Myspace was first to be banished back in 2007 (so 2007!), and now Facebook has met a similar fate. Tinycomb has quoted a Genius:
“Apple Stores have become a regular Internet Cafe, so placing the most popular time-killer [Facebook]of them all on the banned-list will certainly help everyone get a chance to test out the computers.”
Update: Fortune’s Mac blogger says the ban is on a store-by-store basis; many have indeed banned, but it’s not Apple’s policy as a whole.