It might be around in 20 years but I doubt it will ever be any more popular than it is now. It already seems pretty uncool (my grandma's on there but neither I nor any of my friends are) and it's gotten a pretty bad reputation, not many people are going to be too sad to see it replaced.

Shouldn't discourage anyone from doing a web startup though, people are always going to want those sorts of social sharing sites, just maybe not that one.