Scientists at Princeton have modelled online social networks in the same way we look at infectious diseases. They adapted their model using data from MySpace and predict Facebook will lose 80% of it’s users between 2014-17.
Here is their paper: http://arxiv.org/pdf/1401.4208v1.pdf.
Update: a response from Facebook https://m.prod.facebook.com/notes/mike-develin/debunking-princeton/10151947421191849
Draw your own conclusions.