Every Sunday, he sticks the postcards online for the whole world to see. His only request is that the secrets be true and that they've never been told before. Other than that the floor is yours to spill whatever you like, be it sexual, pathological or whimsical.
If the world has a secular confessor, Warren is probably it. He talks of his site as staying pure so as to serve a higher purpose, a notion that mostly means rejecting any advertising, aside from a single link to a suicide prevention site - a cause he has raised US$1000,000 (NZ$1190,000) for.
Ordinarily the secrets he receives are told via decorated postcard, but lately people have begun queuing up to declare them in front of a paying audience.
As you'd imagine the contents of his mailbag can be as powerful as they are weighty. His basement is dominated by a growing pyramid of postcards which he reckons would weigh an easy tonne - he has never counted them, but there must be hundreds of thousands, several more hundreds arriving every week.
"I can hear them creaking," he says from his Maryland home.
"They're just behind me, every postcard I've ever been sent and I've read every single one of them. It's a personal commitment and I think that's how I've earned the trust of strangers. That really is my home address on my website and books... even if that's much to my wife's chagrin."
Just how strongly he now personifies PostSecret was shown by the email he was opening when I called.
It had been sent by a woman who had driven 960km to attend his last event, two days previously. Her sister died four years ago and while sorting out her belongings she'd found an unsent postcard addressed to Warren - reading PostSecret was their Sunday ritual so she decided to hand it over personally.
The email linked to a YouTube message, which he played as I listened.
Which is the thing about his work, it's as voyeuristic as it is anything else and I felt rather uncomfortable listening to her teary thanks for providing a forum where she could let go and move on: "Thank you for helping me heal," she said in closing. He's since posted it on his twitter feed.
People can think whatever they like about his work, says Warren. It's an art project first and foremost, so all reactions are acceptable.
Where it all stems from is hard to say. Warren keeps his own secrets to himself, thank you very much, although he does sneak one of his own into each his books.
He will admit to his fair share of troubles and during his university days became tied up with a Pentecostal church that did a fine line in tongues, testifying and rapturous running up and down the aisle: "I never could speak in tongues, but I learned a lot about personal testimony."
After the suicide of a close friend 10 years ago, Warren volunteered as a helpline counsellor. That experience has helped him deal with the more painful messages.
Instead of seeing them as burdens, he says he feels a "solidarity" with the sender.
As for his art, he says he has a strong - and secret - connection to postcards and experimented with several ways of using them before kicking off PostSecret. One saw him, every Sunday (another constant in his work), tossing a bottle into a lake where The Blair Witch Project was filmed.
Each had a postcard hanging inside. "I'd write these cryptic messages like 'instead of looking for answers discover why you're asking the question' and then leave them to be discovered."
Gradually, as more bottles turned up, their mystery grew until they made the Washington Post and then national television. What on earth did they mean? If the answer was slightly deflating their success only encouraged him to go further.
Then he struck on the idea of getting people to send him postcards instead, so he started handing out thousands of blank cards and inviting people to send anonymous secrets which he'd put online. PostSecret went live on January 1, 2005, perfectly timed for the world's growing love for personal online exposure."I think it's art but didn't see myself as an artist. I was the curator, an entrepreneur, a suburban dad. I never really hoped to achieve anything, so I was really shocked at how it resonated with so many people. I never had any ambition to lead it anywhere; actually I think one of the reasons it has grown the way it has is that I've let it lead me... like with the live PostSecret events, I feel like they have evolved to the point where they are the highest manifestation of the whole concept. I have to go through a definite process to make everyone feel safe and willing to speak, but when it works I get so much satisfaction from it. It feels really meaningful, liberating."
Warren had been reluctant to move into the real world - for one thing he's terrified of public speaking, but he eventually felt he had no choice.
"That was definitely a point where I felt like I had to make a full commitment, I'd become known as the PostSecret guy and I had been aware of that transition happening. It took about a year as I sold my business and went full-time. My wife, she didn't understand so much to start with, now she really likes it."
Though he still refuses to commercialise his website, Warren's books and travelling roadshow do provide an income, although not without a few unexpected side effects.
Making public your home address brings its own obvious challenges - unusual visitors are not uncommon, but he won't go into details - while the nature of what he's being sent has aroused the interest of the FBI and scholars. His long-suffering mailwoman, Cathy, has a following all of her own, even if her efforts have not stopped mail being misdirected to his neighbours: "In a strange way it's like our whole community is coming along for the ride as well," says Warren.
His work was success followed by success until he launched his PostSecret mobile phone app. Between September and December 2011 anyone who bought it could post 100 per cent anonymous secrets and then comment on others' without fear of being identified.
At its peak Warren was fielding 10 secrets every minute, 24 hours a day, from all over the world along with a constant flood of comments.
As you'd guess, your usual internet gutter dwellers came along for the ride to attack, threaten and abuse. A team of volunteers stepped up to monitor all incoming submissions, but the workload proved overwhelming. "No, it didn't go as I'd planned, you can't control who has an app and who uses it and we found the potential for harm when you bring a whole lot of strangers together like that is too much. So it had a short, brilliant life and then we killed it. I won't be trying again."
And that's about all Warren knows of his future. Otherwise he doesn't know what will come next or how he would even go about stopping the train. PostSecret remains as popular as ever; the event he's bringing to Auckland on April 12, and his exhibitions, have given it new life and a means of making money. His difficulty is the degree to which he is identified with it. Would people have the same trust with someone else in charge? Could he even hand it on to someone else? And what would he do with all the postcards?
He really has no idea.
No matter, he's far from ready to give up his window into the human condition."It's definitely made me more open to accepting people's behaviour. We cover a very wide spectrum, but what I see week after week is people searching for that one person they can tell their secrets too. It's a search for that intimacy and the desire to unburden ourselves - I think that's common to everyone at some level - and I think there is something very powerful in finding the words and the image to express a secret. It's about taking ownership, and, yes, it's anonymous but sometimes it can turn out that the most important person you need to tell is yourself."
Which raises the inevitable question: How does he know the secrets are true?
He doesn't care. He even doubts if a lie can be completely false. Let's call it Warren's Law and put it in a bottle: Even when given total freedom to lie you will still reveal truth. Why choose that particular lie? Why take that tone? Why that subject?
Regardless, he says, truth or otherwise makes no difference in terms of it's impact on the reader. The PostSecret chatroom is still reeling from a post last month regarding one woman's rape experience - it wasn't only what she wrote, it was also the way she had decorated her postcard. It outraged, shocked, educated and validated in equal measure.
Warren fretted long and hard over whether to run it or not, but says the ongoing reaction, including from health professionals, more than justified his decision and proved the power of a small piece of cardboard.
"It showed how subversive PostSecret is. You open it up expecting one thing, then the rug will be pulled out from under you, your thinking is fired off in new directions and new connections are made. I believe it's a one-off, and it still feels like what I should be doing."
Frank Warren will host a live PostSecret event at Sky City Theatre on April 12.
