If Facebook were confident that they make a better product, they'd embrace competition as a sort of self-check in the system. They'd spend a day or two to adapt their already-developed you-can-download-your-whole-account feature to allow customers to switch to Google+, if they desire.
But they won't do this. Their reluctance underscores their belief that they don't make a product superior to Google's; if they did, they'd have no problem with these advertisements.
I think it's more nuanced than that. Facebook's reputation and brand equity is perhaps their second most cherished asset (second to their user base). A move like blocking Google advertisements will result in even more bad press when they really need to focus on delivering a better product.
In the end, however, I think Facebook will become obsolete. It's a technology like any other, and I think it has become too big for it to be everything to everyone. Of course, that's just my opinion, and I could be wrong.
But they won't do this. Their reluctance underscores their belief that they don't make a product superior to Google's; if they did, they'd have no problem with these advertisements.