Even though I did recently join Facebook, I'm very much of the same opinion as Jason Kottke regarding Facebook vs. the Internet.
As it happens, we already have a platform on which anyone can communicate and collaborate with anyone else, individuals and companies can develop applications which can interoperate with one another through open and freely available tools, protocols, and interfaces. It's called the internet and it's more compelling than AOL was in 1994 and Facebook in 2007.
Facebook is the new AOL [kottke.org]
I have sort of the same feelings as you about Facebook, except for one thing: they really do seem to have a few things figured out that make it a surprisingly useful friend-management utility.
It does many-to-many communications and the sort of passive sideband stuff (hm, what's my friend's status, who is talking to them) stuff effectively. All this info is stuff I wouldn't really want to see in my mailbox.
Now, none of that is an argument for going out and creating your startup's killer app as a Facebook application: if the app is compelling, I'll still go to your website to use it...