Currently, I cannot recommend any (haven't tried them). For an in-depth comparison see TechCrunch: Nine Ways to Build Your Own Social Network by Mark Hendrickson. This article is from July 2007, but it at least shows what to look for.
See also: Five Best Online Image Editors (2009-07-05)
Keywords: todo, personal, pim, social networking, collaboration, web 2.0, web2.0