still you don't answer if is irreplaceable and why
I thought I did in the first comment. It's hard to get all your friends to move with you at the same time to a new social network. Not that it can't happen, everything is replacable. It's just much harder than replacing google, because with Google I can switch on my own.
That's exactly it. You're probably not seeing how they're different. When you used Google you saw that it's different from Yahoo, but I'm sure many people didn't see the difference. After all, they're both search engines. Same here: although they're both social networks, Facebook is very different from MySpace. When I used Facebook the first time, I got that Aha moment you talked about.