There are a few more sites that I would have included if I had had the time. One is LinkedIn, a great site for business networking, and another is Twitter, which has recently seen a dramatic increase in usage.
LinkedIn would have fit in the same chapter with Facebook. While there are probably far more genealogists on Facebook than on LinkedIn, LinkedIn would probably appeal more to the person who wants a more serious networking experience (without so much emphasis on photos, games, etc.). I see LinkedIn as a very appropriate networking site for professional researchers, librarians, archivists, writers, editors, speakers, and instructors (just to name a few).
Twitter has become the predominant networking site for the short message user. It may become especially handy at helping genealogists connect at major conferences.
If you're using either LinkedIn or Twitter as part of your genealogical life, why not let us know here in the comments?