What others have noted here needs to be considered. It's not the fact that Facebook is blocking ads that point to google plus, it's that they (apparently) have chosen a particular competitor and applied a completely different set of rules that others are not bound to. That, by definition is anti-competitive. I'm not lawyer, and certainly wikipedia should be taken with a healthy dose of common sense, but these are the first two sentences of the entry on United States antitrust law: "The United States antitrust law is the body of laws that prohibits anti-competitive behavior (monopoly) and unfair business practices. Antitrust laws are intended to encourage competition in the marketplace." http://en.wikipedia.org/wiki/United_States_antitrust_law
Antitrust is EXACTLY the same thing that Google is now being investigated for by the department of justice... by promoting an unfair pricing / ranking schematic against competitors by ensuring their own ads are listed higher than those of the competition. (Yelp, I believe).
If I were facebook, would I find a way to stop Google+ ads? You're damned right I would, but I would be subject to the same scrutiny of those who comment here.
This is not a no-brainer. This is a big deal.