HN user
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.
> 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.
One of the most useful aspects of YC are the weekly dinners. You know, face to face interaction with other entreprenuers. It's very valuable. It might be worth while to organize a weekly gathering for entreprenurs working on similar projects.
I'd say that for most people on facebook, it's easier to replace google than to replace facebook. You can always use Yahoo or MSN for search, they might not be as good, but they're good enough. You can use Yahoo maps or Live Earth. And you can use Yahoo email, which is getting really good recently. But it's hard to leave facebook unless all your friends leave with you at the same time to a new social network.