What they have in common is that they have crossed a line to put too much online - in someone's eyes, at least. "You used to be fun," reads the STFU strapline. "Now you have a baby."
Throw some parental preciousness and outright bad manners into the mix, and it can become pretty toxic. If there is a way for a parent to behave badly, you name it, it will have been logged by Koenig and her followers, who flood her inbox with horror stories and screengrabs.
Mummyjacking - using others' unrelated observations as a chance to weigh in on parenthood - proves a particular bugbear, usually involving the offender telling a childless friend to "try doing that when you're pregnant and have three children".
One well-wisher congratulates a newly qualified doctor with: "How exciting. Now you need the title 'mummy'."
Sometimes, it is a picture that is worth 1000 words. A memorable Facebook photo is best summed up by Koenig: "The baby is crowning in this picture! Why would anyone put that online?" A veil of mystery will be drawn over the "Placenta Shake" episode, meanwhile, in which a new mother, keen to get her vitamins, got busy with her blender.
But for Koenig, worst of all are the Sanctimummies (and Sanctidaddies, for that matter) "who think that their parenting is better, or go on these ridiculous rants". Winning a place in her hall of fame is the mother who demands a quiet rule for public bathrooms after strangers woke her sleeping baby.
"Two EXCESSIVELY LOUD FLUSHES LATER I was holding a wailing infant!" she roars - only for an online contact to capture her outrage and send it to the blog.
Nor is it an American phenomenon, with Koenig tapping into tales of bad behaviour from all over the world. "The only differences are the language quirks," she says. "Everything else is exactly the same."
Proud parents are, of course, nothing new.
But what is a recent development is the myriad ways they can now update friends, family and relative strangers on Baby Harry or Harriet's sleep patterns, eating habits or bowel movements.
It was this that prompted Koenig to start her blog just over four years ago, while working as a freelance editor for a network of humour blogs.
"One day, somebody posted upwards of 10 or 15 updates about her baby's fever changing - going up and down, the temperature and everything," she remembers. "So I sent an email to a friend and said, 'Am I the only person experiencing this?' And - she has two kids - she wrote back with a screenshot of her friend's annoying update bragging about her child. So I started thinking, Okay, well, if she thinks this too, then maybe other people also agree."
They did - and were grateful for a place to vent. Other examples started flooding in from the second day of the blog's existence, adding up to many thousands of tales which have been logged (and anonymised) over the years.
"It kind of took off right away," says Koenig. "I have not gone a day without getting submissions since then. It's rude to say behind your friends back, 'Ugh, why is so-and-so updating 15 times' ... [but] if it's a stranger, on the internet, it's just a respite."
Having started out blogging anonymously (only close friends knew about the project) Koenig last year broke cover ahead of the book's publication this year in the US.
"I thought, 'Everyone's going to hate me'," then later worried she might be the focus of their hatred, rather than the content of the book - and she "didn't want that either".
Certainly, although written with wry humour, her blog has sparked some vehement reactions - notably from one man who "went into an elaborate story about how he would enjoy having a hand in my death, basically. That was interesting."
But Koenig, who isn't a mother herself, says she gets more emails from parents than non-parents writing that they love what she does, some even using the site as a "what not to do" guide. "A lot of parents email me and say, 'I'm already changing my daughter's diaper multiple times a day, when I go on Facebook I don't want to see my friend's daughter's diaper contents,"' she says.
Touchingly, she also hears from people struggling with infertility, who say they come for some light relief from the tide of baby updates they face online.
Certainly, on Mumsnet, the all-conquering British mothers' site with more than 50 million page-views a month, a sizeable movement for more restraint is in swing. "Someone I went to school with puts up pictures of poo - nappy explosions, results of nappy-free time," frets one commenter on the Mumsnet site. "Send them to STFU Parents," comes the reply.
But it is a divisive issue, provoking accusations that those behind the complaints are guilty of "competitive apathy". For one mother, posting links from the STFU site is the real social media solecism, meriting her blacklisting "one woman [who] was constantly posting anti-children and anti-parent diatribes".
What such reactions confirm, of course, is Koenig's success in tapping into a source of irritation, which just did not exist in the same way a decade ago.
In fact, not only has social media enabled parental oversharing, but can actively encourage it, according to Sandra Wheatley, a social psychologist. For many, childcare will come as a break in a career involving working with computers, expressing themselves in written form, "so in a way social media can be a way of continuing what they used to do" - as well as get some reassurance that they are doing their new job right. But that can come at a price.
"We all know, ultimately, whether people are interested in what we are saying," she explains. "The difference with social media is that you don't have somebody looking at you askance, so there's no instant feedback, somebody saying, 'Please, no!' People are reluctant to put that in writing, so you are reliant on your own boundaries."
And if you are a thrilled new parent, your grip on these may be quite fragile.
"I think we're all naturally inclined to share a little more information than our friends want to know - it's just that with parents, they really don't know where to draw that line," says Koenig. "Their lives are completely altered, something has changed in a very dramatic way and they're in love with their child; they're seeking attention, or sometimes it's just to commiserate with friends." Unfortunately, that means "for whatever reason, parents are the worst".
Many, aware of this, are censoring themselves quite actively. Telegraph columnist Bryony Gordon, a new mother to baby Edie who arrived in April, says the danger is that Twitter and Facebook can be your "only outlets to the wider world" in the early blur of feeding and sleep cycles.
She finds it faintly depressing that some people post solely "about baby slings and colic, 'oh, we took little Johnny out for lunch and he did a wee and a poo'," but sees how it happens. "I have to hold back! I just can't tweet any more, as all I tweet about is stuff to do with babies. I'm only vaguely aware that Andy Murray won Wimbledon."
Her solution for those fed up with tell-all parents? Set your social media to filter their posts - "for the next 18 years".
Still, the pressure to reveal all does lessen once the initial shock of motherhood wears off, says Helena Curran, a publishing executive and mother to 1-year-old Arthur. "Now I just share a really nice picture or something that's funny, like a bad hair day."
Those parents who do splatter all and sundry with updates, are "covering up a bit," she suspects, "professing to be the perfect parents by documenting everything and going on about it all. It doesn't ring true." As for Koenig, "she's bang on! I don't think she's anti-parents or anti-sharing your life online, I think she's just against bonkers parents."
By now, Koenig herself has established some strict ground rules for any parents in doubt about how to avoid falling into that category. "Anything with bodily fluids is definitely overshare," Koenig says, with a laugh. "Anything that comes out of your child is probably something that doesn't need to be seen on the internet.
"Other than that, I think if you're editing yourself, then you know. If you post, you know, 400 pictures of your labour and delivery, you probably know that they're oversharing." It is just common sense, as she sees it, but the danger is people choose to "ignore that little voice".
She is not, she insists, a child-hater, and sees babies looming in the relatively near future with her long-term boyfriend, with whom she lives.
"Sometimes people misconstrue the site and they say, 'Apparently people don't like it when I post cute pictures of my children' - and that's not what it's about all," she says.
"It's just if you post 20 pictures of your children every day, then I think you might be going off path."
From her own output as a potential parent, she promises "no pictures of poop" - a feat she expects to achieve without a struggle. "I'm a pretty private person. I don't feel compelled to share details about where I'm eating, who I'm with."
She is looking forward, she says, to "not sharing every detail of my child's life".
STFU, Parents: The Jaw-Dropping, Self-Indulgent And Occasionally Rage-Inducing World Of Parent Overshare (Perigee Books, $20) is out now.
