Declaring your relationship status on Facebook is a rite of passage for many of us. Now we've heard that Facebook can actually be a relationship killer, with one in five divorces involving the social networking site. The American Academy of Matrimonial Lawyers says that 20 per cent of divorce cases cite Facebook as a reason for the marriage's failure. Additionally, 80 per cent of divorce lawyers have reported a spike in the number of cases that use social media as evidence of cheating. It seems that couples are using the site to send flirty messages - just not to each other.