I can definitely see Facebook being an additional tool, the same way buying ads on AdSense is. I just think that if one is losing all of their traffic and community because something happened to their social networking element, they're doing something seriously wrong. The goal of the Facebook presence should be to funnel people into the real service or community or information on the real site and not to supplant it (and once they're on the real site, not to have all your services like forums handled by Facebook Connect service widgets which places you in a similar situation, too).

I'm speaking from limited experience, of course, and with a service that established itself long before social networking even existed. I think the premise is still sound, though.