That isn't at all what the article said. What it says it that one in five (20%, if you will) divorce cases cite Facebook as evidence of either infidelity or irreconcilable differences. This isn't evidence that people cheat more or divorce more, just that it's a whole lot easier to access someone's Facebook account than hire a private detective.
“The survey reflects the findings of UK law firm last year showing that 20 per cent of its divorce petitions blamed Facebook flings.”
That says 20 percent of UK divorces handled by one firm were from Facebook flings alone.