Google solved the problem of "find me the best webpage" in a clearly better way than earlier tools. Facebook solves the problem "find me a friend to chit-chat with" no better than any listserv I am on, and notably worse than my cell phone. So I think Facebook is in more long-term danger of running out of runway. Google reduces annoyance for searchers as it spiders more websites, while Facebook increases annoyance for subscribers as friends add other friends.
I have my doubts about Facebook's business potential, but I don't think it's fair to compare them to listservs or cell phones.
For one thing, Facebook is clearly not a chat platform, nor is it a venue for in-depth discussion (try doing it sometime, the limitations become apparent).
It's a way for you to keep tabs on friends in a passive way without engagement - which is something that I think will only increase in the coming years, for better or for worse.
Where you only had time for a close circle of friends before, and knew essentially nothing about your many brief acquaintances, now you can keep a much better track of your social network without nearly as much effort. Your old coworker John? He's at Google now, and you didn't even need to email him to find out - in fact, you didn't have to ask anyone at all. It allows what used to be your "peripheral" social network to play a larger role in your life than it used to. Think of it as a tool to improve your general social "situational awareness".
[edit] Another strength of Facebook is that you don't have to participate to derive value. I don't post status updates to Facebook very much, but I enjoy keeping up with friends who do. They're in a unique situation where a small group of hardcore users can keep perhaps an order of magnitude more casual users coming back for more.
a way for you to keep tabs on friends in a passive way without engagement
That does sound like an apt way to sum up what Facebook promises. I'm not sure
a) that's worth much money to me, even if the money is extracted from me by responding to advertisements once in a while,
and
b) whether another company couldn't provide that functionality less expensively and less annoyingly and thus have a way to beat Facebook in the marketplace.
