As reported by web monitor Hitwise, this sites have been growing faster than the web itself. Between 2006-2008, use of these types of sites has increased 889%. The report was based on data from eight leading Q&A; sites, and of course due to the growth, there are new Q&A; sites popping up regularly now on a global scale.

Top sites according to the report: (note this is growing older)

Yahoo! Answers 74.05%
Wiki Answers 18.35%
Answerbag 4.51%
Ask Metafilter 1.8%
Askville 0.85%

Now, this can be a great way to generate back links and get people to bookmark you and really increase your inbound links and exposure on the web. If you have subject matter expertise, you can post to these sites and increase your exposure for your company and website.