Ecademy
I've been on Ecademy for a long time. It is based on an early version of Drupal, and is a pretty good group of people. It has a strong focus on ebusiness and particularly in England.

Ryze
I've been on Ryze for quite a while as well. It seems very business oriented, particularly in the States. A lot of people are on both Ryze and Ecademy.

LinkedIn
This is an interesting, fairly business related, social network. I use it from time to time.

Plaxo
This is a nice way to share contact information. Unfortunately, the only way to get any of this information seems to be through contact cards, and not webpages.

Friendster
I connected to Friendster a while ago. I don't go there that often. Recently, it has started having a lot of ads.

Tribe
I probably joined Tribe shortly after I joined Friendster. I still go there from time to time. I also get emails from a couple groups there.

Orkut
This is the Google supported online social network. Fairly nice interface. I go there from time to time.

Multiply.com
I just set up an account there today. It looks like it has some nice features. It will be interesting to grow my network there and see how well it really works.

Huminity
Unfortunately, Huminity doesn't work well on Windows NT 4.0, which I am still running most of the time, so I don't use it very often.

There are probably other networks that I am on, which I've forgotten right now, but this gives a good view of the networks, and of my persona on these networks where they are publically accessible.