Interesting question...I have just begun to research this.  

MIT Technology Review tests several question and answer search engine services.

They reviewed and ranked AnswerBag, Askville, Live QnA, Wondir, Yahoo Answers and Yedda. So how did they rank? Out of a total 12 points, Yahoo Answers scored 11 points, coming out tops.

Here's the rundown:

1. Yahoo Answers (11 points)
2. Live QnA (7 points)
3. Askville (6 points)
4. AnswerBag (4 points)
5. Wondir (4 points)
6. Yedda Answers (4 points)

Hitwise posted
figures and data to show market share of the different Q&A search engines.

• Yahoo Answers with 96%
• Answerbag with 2%
• Windows Live QnA with 1%
• Wondir with 0.7%
• Askville with 0.11%
• Yedda with 0.02%

Linky Links: 

http://www.metafilter.com/

http://answers.yahoo.com/

http://qna.live.com/

http://wiki.answers.com

www.wondir.com

www.answerbag.com

 Enjoi