I suspect survivor bias. If you believe there is a billion people on Facebook (even if it's wrong by order or two of magnitude, doesn't matter), several millions of them must be gay. Some percentage of them would be closeted gay. Some of them would have friends that visited or liked "coming out" pages. So if Facebook just displayed random page which your friends liked - without knowing anything else about you - it would inevitably happen that somebody who is gay and thinks about coming out would see such ad sooner or later and would be creeped out about how much Facebook knows about him.