There are mixed opinions about the debut of Facebook in the Q&A arena. Some people consider them useless.
I believe that here we are observing a complete new shift paradigm for search: a massive amout of users can be ranked according to their contribution for a specific activity. In this case, Q&A productions.
Traditional search engine (Bing and Google) can rank objects such as web pages, but they cannot rank the authors of such objects because they do not know them directly. Instead, Facebook can rank 500m of users whenever they produce an answer by taking into account any single action that each user peformed in Facebook before producing that answer (status updates, searches, link posted, etc).
Communities are the ranking engine with all the rich information produced by them (not just the anynimous links produced). Wow, what a change!