Right, Facebook is a business, and they were being paid to run an advertisement. It turns out that the ad (albeit indirectly) is for their competition. Blocking on the presumed basis that the advertisement may encourage users to leave Facebook is bad business. The advertisement is legal and honest, but the problem is that it 'might' cause people to leave Facebook. I think we all 'get' that Facebook is running a business, but this practice is anti-competitive.