I see this as part of the App-ification of computing. People forget what "general purpose" computing is, and figure that because FaceBook has a particular aspect of the web locked up right now, that's the only way to do it.
Yes, there are network effects, but those network effects didn't prevent FaceBook from supplanting AIM/MSNMessenger/ICQ etc. (yes, there are people still using those tools, but not nearly as many as in their heyday), and they don't have to prevent the next tool from supplanting FaceBook.
I think the 'net is suffering from an overreach of Branding - we need to turn FaceBook into the next "Xerox" or "Kleenex" so that when you use Bing to google a way to notify all your friends - real or imaginary - you get options, and can choose the one that's best for the situation.