If you go for a borader definition (User involved/User content/Communittee applications) the number is closer to 30 (shopping.com qualifies as well...)

Check this blog for some examples: http://www.israelwebtour.com/blog/

(BeInSync , Fixya , Foxytunes, ILCU, inter8ing, Kasamba, MyHeritage, RadarSync, Skylow, SpearCast, Trusted Opinion, ViewScore)

Go2web20.net is not a technology company, but it ranks others and can help as a reference guide

Some names have been mentioned by other replyers here, other names you can look into are Gigya, InterBine, MuseStorm, Olivelink, CoMagz, g10L and Mobixell