Started by the controversial librarian JP Porcaro who was recently named LJ mover & shaker 2012, it is an extremely active Facebook group.
Either group doesn't really have any specific theme beyond the fact it must be library related so pretty much everything goes from asking questions, sharing articles or comments. For most facebook groups you have to be either invited by facebook friends who are already members or you can apply and a moderator of the group will approve.
Q&A sites for librarians to ask their librarian peers aren't new. eg. Libcatcode . This is different in that this is on the well known stack exchange network platform which is popular with coders to ask programming related questions.