as facebook and "the internet" merge, the reasons for having an account in the first place recede. facebook originally made it easy to connect to people, but as advertisers, politicians, and other interested parties find various ways to get themselves on your feed, you'll have to wade through more and more muck to actually connect to people in the way you originally wanted, which subverts the whole point of being on there.
so in my opinion facebook is at its apex right now. they'll plateau in the immediate future if they havent already. i dont think they're going to fade into irrelevance, simply by virtue of inertia, but its hard to see anything really exciting in their future. and if it winds up as just a way for the government to tie your ip address to your actual name (see GG here) , some negative PR could push people on to some other system.
in their battle with google for the internet, i'm backing google.