There is a real threat, but Google could answer to it in a much better way: By positioning Google Plus as "the open web alternative to Facebook". To do so, they should make G+ the identity/social graph platform on which third parties want to base their own services instead of FB connect. I'm sure they could do that, because Google can profit from people going around in the web, while FB needs to keep people as much as possible inside its walled garden.

Unfortunately, it doesn't look like G+ is moving towards that direction.