How to find Experts with the Social web?

• Wikis: Wikipedia and Knol: authors are often well-known experts
• Blogs: use specialized engine like Google Blog Search
• Presentations: use the search form on slideshare
• Twitter: TOPSY returns results showing you who has been speaking about your keyword the most, on Twitter
• Linked-In: use the Advanced search to type keywords or browse topics groups
•  Experts Directories (Q&A service maintained by volunteers)
  • AllExperts.com: Health
  • JustAnswer
•  Whos Talkin searches across many different sites including blogs, social networking sites, news sources, social bookmarking sites, and video and image sites

My opinion: these free tools, whatever their quality, cannot be compared with an extensive search on paid databases (literature, patents, etc.) to identifiy opinion leaders, etc.