The unfortunate part of people using Facebook for queries is that they are lost after the moment. There's no ability to search.
I am in a few "genealogical" Facebook groups -- co admin some. However, I find I am always pointing them to the archived geni sites that they can search and post for others to find later.
I find that Facebook is best for connecting today's cousins... via searching the friends of relatives. Or searching to see if someone has an account. Far more productive than posting my particular query in a randomly related (area?) group. Seems that more often than not it is newbies beginning some of the groups and newbies posting to them.
I am not really seeing value in posting queries at Facebook.