Rough chronology of social networking websites I joined (might have missed out some in between):
1. Six degrees (allows you to see how you are connected to people until six degrees)
2. E-circle with group forums? (Forgot!)
7. Twitter (micro-blogging)
Check out this top 10 social network sites review for 2010.
Anyway, why Facebook is such a hit?
I suspect it's the apps with all the different kind of mindless yet addictive games. There's also the ability to find old schoolmates and friends. And not forgetting, able to be nosey to read and see pictures of your friends!!
Take a look how South Park has made fun of Facebook. In fact, this morning, the deejay on the radio was exclaiming in horror if Farmville (an application in Facebook) becomes a movie! lol