It would be very funny to see the next big startup beating facebook using facebook's Open Source tools. I mean, to scale that high, you would be silly not to. After all, Diaspora's betting on MongoDB (wonder why they didn't go with Cassandra, hmm? :-p)
As about the future, facebook will continue to make the web facebookable, the "Like" is only the beginning. I wouldn't be surprised if facebook integrated loopt to make real places "Like"able and have check in's as a core feature. facebook sign-in will continue to have more impact and it's likely that facebook may even dominate commenting using a Disqus like approach.
It's hard to imagine tectonic shifts, but they happen in technology pretty often. And we have all the ingredients for a shift. The only thing left that's needed is a well-played challenger, and we'll get several by year's end.