Facebook Phenomena

Facebook is truly phenomenon. Most people will love Facebook, due to it's simplicity (even though it can be complicated), easy registration, and lots of application available and the best of all, it's free (some application may require you to buy something if you want to get more items or features, but it's optional features since you can still play without having to buy those items). You can also participate on the development because they opened their API for developers.

One thing that i don't see in Facebook is Blog, but i guess it's not very crucial, because there are already several blog services which are also free, such as WordPress and Blogger. Wiki is also not crucial in my opinion, as there are already Wikipedia.

I wonder what kind of evolution which will make people switched from Facebook? Any ideas?