LinkedIn (my profile) LinkedIn is one of the premier business networking communities online today. I'm constantly surprised by the quality of contacts I make through this network.
Google (ego surfing: Web, Patents, Blogs, Glists, Citations) Developers at Labs.google.com are doing some of the best and brightest work on the Internet. If you don't have a Gmail account or a Gapps domain, I highly recommend getting one. Ego surfing is what computer scientists do with their spare time--seeing how many times your name comes up on various search portals. Like having bragging rights to the longest usable email address, large numbers of results gives you big cachet in the geek worlds.
Renkoo (my profile) Renkoo is a new and unique Internet experience for blending an over-abundance of offline and online communication centered around people activities into a manageable social network. As Adam says, everything's inter-twingled.
FoRK, 4K-Associates (Xent, 4K) FoRK stands for friends of Rohit Khare. Decades from now, Internet researchers will have few clueful archives to understand the process which technology was conceived, understood, adopted, and evolved. At one point the list contained the who's who of the smartest Internet technologists who, taken together, probably had a small, but meaningful effect on the lives of everybody on the planet.
Friendster (my profile) Friendster was the first major social networking site. They grew from a handful of people to over 50 million users in a few short months. Their problem, however, is that they grew so fast that their core community grew uncomfortable with the international crowds from asia, brazil, and eastern europe. Culture clash ensued and everyone went to Myspace and Facebook.
Myspace (my profile) Myspace is one of the most successful worldwide social networks. It's one of the few with an actual revenue model. They are expecting 300 million users with the launch of Myspace China (2008). I had a chance to be one of the first research scientists to work at Myspace R&D which I passed up. The jury's still out if that was a good decision or not.
Facebook (my profile) Facebook is one of the world's largest startups. I signed up for their beta-service (pre-launch) back when they were focused solely on school networks. It's street name is "crackbook", but continually amazed at how much luridly private information people post in their "what are you doing now" box.