Ok, I've never read their terms of service, until now. I'm guessing they have provided credit to the actual authors with a link to the original article and included a licensing notice? (not on Facebook)
http://wikimediafoundation.org/wiki/Terms_of_Use
Facebook could have scraped Wikipedia without asking, but as it happens, there is an agreement in place. I happen to work at the WMF, but don't ask me for more details, I'm not that privy to them.
This thread might help.
http://www.gossamer-threads.com/lists/wiki/foundation/194087