How should Facebook combat open social networks as a business threat? Two possibilities. One, just be the better product. Use your head start to keep adding the features that people want in their social networking. Two, allow one-way interaction. You could view anyone else's external feed integrated with your Facebook page, but nobody can view Facebook anywhere but Facebook itself.
Mokalus of Borg
PS - So far, the threat of other social networks to Facebook is minimal at best.
PPS - And they're already doing one-way interaction of a sort.