Facebook, which was first blamed for encouraging illicit encounters, is being increasingly cited as an evidence while seeking divorce.
Facebook has a divorce page where people can post comments about what they are going through, and 5,700 people have said they like the page.
Lawyers are seeing a steep rise in divorce petitions involving Facebook as they struggle with the post-Christmas increase in marriage breakdown in Britain, The Times reports.
Emma Patel, head of family law at UK firm Setfords, said: "There is a distinct trend in social networking websites being cited in divorces, almost as a virtual third party. Facebook features in 30 of the petitions I have seen since May, which is nearly all of them."
She said that the huge popularity of sites such as Second Life, Illicit Encounters and Friends Reunited were tempting couples to cheat on one another, The Times reports.
Facebook pages were increasingly being cited in evidence as "unreasonable behaviour", she added, including flirtatious messages or e-mails and chats of a suggestive or sexual nature.
It is estimated that 14 million Britons regularly use social networking sites. The popularity of Friends Reunited a few years ago was blamed for a surge in divorce as people contacted old flames, The Times reports.
Patel said: "The situation has deteriorated so badly that we advise feuding couples to avoid these sites until their divorces are settled."