I think we can choose questions/answers with highest votes or highest number of views and then to choose among them to used for Advertising. Advertising should not be limited to just Stack Exchange sites, we should expand advertising/commenting/linking on other Hinduism sites/blogs. I can remember that I have seen many misleading articles on a site (www.SpeakingTree.in). It would be best to comment over there linking to the correct answers which are present on Hinduism SE.
Regarding the Christian SE and other religion SE, I don't think that it would be the right place as every religious person considers his religion on priority and would not like to visit into much details of other religion. On the other hand, the person viewing any blog or site consisting of Hinduism, is already interested in the subject and can get more enlightened by coming here and getting correct facts as per scriptures.
Also, not to forget, please do mention in the ads about the fact that the answers here are as per what is mentioned in the scriptures which is along with the proof texts, so that should get more attention.