Basically, for the subset of things where having the most people you know on the site is more important than the fact that the interface of some other site is better, facebook may win out.
I know people who have resisted joining any social network that are succumbing and joining facebook. These are people who are not going to be signing up for half a dozen accounts on different specialty sites anytime soon. If facebook does it, they'll likely use facebook for it. If I want to keep in touch with them, or invite them to an event, facebook is the easiest way to do so. This means I'll use facebook more for these things, even though (unlike some of my friends) I'm perfectly willing to sign up for all sorts of specialty sites.
Basically, for the subset of things where having the most people you know on the site is more important than the fact that the interface of some other site is better, facebook may win out.
I know people who have resisted joining any social network that are succumbing and joining facebook. These are people who are not going to be signing up for half a dozen accounts on different specialty sites anytime soon. If facebook does it, they'll likely use facebook for it. If I want to keep in touch with them, or invite them to an event, facebook is the easiest way to do so. This means I'll use facebook more for these things, even though (unlike some of my friends) I'm perfectly willing to sign up for all sorts of specialty sites.