I think the big mistake Facebook is making here is showing fear. Every time they close down a path for contact export, spread FUD (e.g. Zuckerberg leaving Google+ for "privacy" reasons) and so on they make it very, very clear that they're terrified of Google+'s potential and they're going to do everything they can to stop it.

The problem is that Facebook isn't the late-90s Microsoft. They don't have the raw monopoly power to stop people from leaving for Google+, they need to persuade them. And every move they make to try and force people to stay (instead of enticing them) makes more people want to leave.

Perhaps the Ad ban story isn't what it seems, I'll reserve judgement on that. But it certainly fits the Facebook's overall Google+ pattern.

Contrast this with Twitter's reaction. Google+ circles can be used asymmetrically, so they're equally in danger. However, unlike Facebook, they've ignored Google+, at least AFAICT. When Google+ was announced I actually thought Twitter was in the weaker position, but I'm starting to think I was wrong.