I think they have legs. I mean, I think people will still use social networking in 5 years. And they may do it primarily on Facebook. I'm not trying to say that it or Facebook are a fad, or that Facebook isn't very good at it.
I just don't think their value at the time will even be 1/10th of Google's because of the distinction you accurately summed up.
They almost can't monetize like Google. Google is active. You go there because you're looking for something specific. It's really easy (now) to make money off of the guy who Google's for "ipod".
On Facebook, you're relaxing. You're not in action mode. It's going to be hard for them to come up with ways to monetize passive traffic. The best anyone has done in offline media is show branding ads. This may work, and may even make them profitable, but it won't make them Google.
I just don't think their value at the time will even be 1/10th of Google's because of the distinction you accurately summed up.