"This is exactly what happened in the cat torture case. One of the people named was dragged through the dirt and that person was not involved with the incident at all," Mr Cook said.
Another example was a home invasion in Runanga last weekend, in which a drug-crazed skinhead burst in on a family, who he did not know, and terrorised them before fleeing.
In that case, the names of suspects were bandied about on Twitter and Facebook, and at least two of those named were innocent.
"Innocent people are being named because some of the information on the site is clouded and this is the main problem," Mr Cook said.
He said there was a public expectation that when a suspect was named on Facebook police could arrest them straight away.
"We cannot. We have to go through the evidential process."
He said Facebook users should avoid "splattering" names of suspected criminals online until they know for a fact that that person had been convicted of the alleged offence.
"I would urge people to think before they push the button." Mr Cook said police acknowledged that social media was now part of modern New Zealand culture, and that Facebook and Twitter, when used responsibly, could be useful crime-fighting tools.
But too often they were also used to hinder or even frustrate investigations.
"We have our own site, which gives us a way to get news out there smartly, as well as receiving information quickly, which helps us solve crimes."
West Coast lawyer Eymard Bradley was blunt: "Facebook is a terrible thing".
He advised his clients not to go on Facebook at all, or at least until their cases had been through the judicial process.
The trouble was there were no limits to what people could post when using sites such as Facebook, Viber and Twitter.
"When people use Facebook things get ugly - no one thinks about the consequences," Mr Bradley said.
"There's defamatory stuff and in a criminal's situation a lot of the time everything is out there on the worldwide web before they can even get any sort of fair trial."
He was blown away by some of the allegations he saw on Facebook and how the site was being used by some people.
"There needs to be protocols set up.
"When things on Facebook are ugly they get even uglier. Social media sometimes gets really nasty and people go for the jugular. Some of it is hideous."
