I suspect Facebook will be around for a very long time. It has partnered with many companies and internet sites. Presently there are over 900 Million in the world using Facebook. Businesses are looking for ways to market to those people based upon information they volunteer about themselves. Yahoo, AOL, and even Hubpages offers one the option to log onto their websites using Facebook.
In comparison Myspace was nothing more than a vainity excercise for people to have their own semblance of fame. Often times these people were adding music to their sites, promoting parties, concerts and events...etc It was child's play compared to where Facebook has taken social networking.
In my opinion Facebook is to social networking what Microsoft is to personal computers. The future may be bring "better" solutions but Microsoft and Facebook have so much of the market (married to them) that other companies will be happy just to get a small percentage or secretly hoping to be acquired by them.