Facebook aren't infallible. Nearly every social media went parabolic growth so far. It's inherent to social media. It growths while the network effect encourages people to talk about it and join it, it wanes when interest fades and people you don't want to see end up there, and you grow tired of seeing them, and as people who you joined to socialize with leave, you leave too.

Facebook will eventually fall. They will desperately try to buy their competition. But the inherently parabolic behavior of social networks will catch up with them and eventually one competition will not agree to be bought.

The most interesting part is that those networks don't realize their inherently parablic growth and usually ending up behaving like they are still growing exponentially at the peak of their growth, quickening the decline.