There are a large number of Facebook-based groups on the topic of genealogy. Among these are:

Irish Genealogy (over 3000 members)
British Isles Genealogy (over 3000 members)
RootsWeb genealogists (over 1800 members)
Random Acts of Genealogical Kindness (over 1000 members)
Tracing the Tribe - Jewish Genealogy on Facebook (over 400 members)
Technology for Genealogy (over 400 members)

And those are just some of the more popular ones.