Well, I think the one thing about social media is that is has to constantly change to stay popular. (Attribute that both to the "social" and the "media" part of social media.) MySpace is on the decline, as we have seen. Unfortunately, I think it was/is plagued by performance issues, spam, and the overall uniformity of the website. They had myspace page themes and such, but the actual content never seemed to feel all that dynamic. (Paradoxically, Facebook is quite static with its design, but is quite dynamic with the content.)
As much as I like Twitter, I think it will wane. However, I think there will be something new waiting in the wings to take over for Facebook and Twitter. One thing that Twitter picked up on (and Facebook to a large degree) is that it is all ultimately about the interaction between people. The next company to grasp and deploy a website/application with this in mind will do well. I think it will come in a different format, but I admittedly don't know what. If I did, I wouldn't be sharing it with anyone, either.
