New research released Monday shows that Facebook is causing an alarmingly high number of divorces in the United States.

Other social networking sites, including MySpace, Bebo and Twitter, also featured heavily in the sample of 5,000 divorce papers studied by Divorce-Online.

The most common reason is spouses engaging in saucy sex chat with their online friends.

Almost one in five online divorce petitions dealt with by one firm cite Facebook in one way or another. Facebook is the largest online social networking site in the U.S. with over 100 million users, and it has over 350 million active users worldwide.

Mark Keenan, Managing Director of Divorce-Online said I had heard from my staff that there were a lot of people saying they had found out things about their partners on Facebook so I decided to see how prevalent it was.

I was really surprised to see 20 per cent of all the petitions containing references to Facebook,” Keenan said after he found 989 instances of the word “Facebook” in 5,000 divorce petitions.

The most common reason seemed to be people having inappropriate sexual chats with people they were not supposed to.

In the US, divorce lawyers openly admit to searching social networking sites for evidence of infidelity or wrong-doing in a bid to strengthen their case against an errant spouse.

In 2008, Amy Taylor from the U.K ended her marriage after discovering her husband had been having a virtual affair with someone in cyberspace he had never met. The 28-year old sought a divorce from husband David Pollard, after discovering he was sleeping with an escort in the game Second Life, a virtual world where people reinvent themselves.

Looking ahead, Keenan believes that the general divorce rate will only rise in 2010 with the recession taking the blame.