Facebook does not actually do anything particularly well. Indeed, I'd go as far as saying they succeed in spite of themselves. Their interface is busy. Their app is always spamming me. The messaging infrastructure sucks. They have confusing privacy policies (and underhandedly change them). Their photo management is weak. I'm constantly being petitioned for scammy quizzes and second-rate games. The list goes on...
The only thing Facebook has, and it's tremendously valuable, is the social graph. They really were just at the right place at the right time.
Over the next few years people are going to gradually decide that dealing with all the shit and cruft of Facebook isn't worth the diminishingly interesting experience of half-heartedly stalking the people you thought were fascinating 10 years ago. And since the value of Facebook is the social graph - once the graph starts contracting, it will do so at an accelerating rate.
The social web will likely continue to be a growing plain, but it won't really get interesting until it starts dealing in standards and decentralization.