MySpace, Facebook, Twitter...what's the difference?

So I have this friend John, who started a blog a while back called Defending the Doctrines of Grace. It was a nice blog about books he'd read, local area information, stuff about Vermont, his faith and walk in the Lord, his running ministry, his family, you know, a nice personal blog! Well, after having discovered Facebook, he has all but abandoned his blog. What's up with that?? What's going on over there at Facebook that has caused him to lose interest in his writing? He now uses every opportunity he can to throw it in my face, that I'd know this or that 'if only I would join Facebook too.'

This got me thinking, what *is* the big deal with social networking websites? I mean, do you have to have a profile on each one? What are the differences between them? How much time do you devote to updating your page? Do I really have the time or the desire to let everyone know my minute by minute stats throughout the day? How in the world is there enough time to visit everyone else's pages that are in your 'friends' list? What is the point? Am I missing something? Isn't this just another temptation to distract us from what we ought to be doing? Isn't this just another temptation to seek out people better left unfound?
Someone please tell me, what social networking site/s do you use and why is it better than any of the other ones. Also, how much time do you devote hourly, daily or weekly to interacting on them? How is it better (or is it?) than blogging? What is 'the point' for you?