That is what a couple of researchers think:
Facebook is like an infectious disease, experiencing a spike before its decline, according to US researchers who claim the social network will lose 80 per cent of users by 2017.
Two doctoral candidates in mechanical and aerospace engineering at Princeton University made their claims in a paper published online at a scientific research archive, but not yet peer-reviewed.
Based on the rise and fall of MySpace, John Cannarella and Joshua Spechler say that Facebook, the largest online social network in history, is set for a massive fall. [ABC]
You can read more at the link but the only way I see Facebook losing 80% of its users by 2017 is if a better service comes around to compete against Facebook.