Facebook is different from MySpace for exactly the use case of keeping up with your friends. Facebook is the only social network so far that rigorously enforces real names, identities and relationships through both social design and brute force. MySpace has degenerated in to a social game to rack up more points in the form of 'friends'. They are completely different, which is why Facebook is catching up to MySpace so quickly and MySpace is sinking.

Right, this is the "aha" one of the parents mentioned. The major problem with the internet is the lack of accountability, accountability is necessary to form trust, and trust is necessary to form good social networks.

v important: "real names, identities and relationships"

> I don't see what is any more irreplaceable about Facebook than MySpace.

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.

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.