It used to be the tell-tale lipstick on the collar. Then there were the give-away texts that spelled the death knell for many marriages. But now one in five divorces involve the social networking site Facebook, according to a new survey by the American Academy of Matrimonial Lawyers. A staggering 80 per cent of divorce lawyers have also reported a spike in the number of cases that use social media for evidence of cheating.  Flirty messages and photographs found on Facebook are increasingly being cited as proof of unreasonable behaviour or irreconcilable differences. Many cases revolve around social media users who get back in touch with old flames they hadn’t heard from in many years.  Facebook was by far the biggest offender, with 66 per cent of lawyers citing it as the primary source of evidence in a divorce case. MySpace followed with 15 per cent, Twitter at 5 per cent and other choices lumped together at 14 per cent. The survey reflects the findings of a UK law firm last year showing that 20 per cent of its divorce petitions blamed Facebook flings.(MailOnline)