A social bookmarking tool.
Care2 Connect is a popular networking tool for members of the nonprofit community meet and plan group-related events. It's completely free, and because you choose who is included in your network, you don't get spam or other undesirable unsolicted messages.
From frappr.com:
Frappr is an online tool that lets you map out the city where you live, work, vacation, or anything else! You can then share your unique URL with friends and find out where the rest of them live and work in relation to everyone else. It's great for keeping in touch with friends, families, and groups! You can also share photos, private message them, or leave comments on their MyFrappr homepage.
Friendster has more than 27 million members. It's a very friendly (!) generalized social networking tool. Popular with the kids.
Furl is a very useful and well-organized social bookmarking service.
An H2O Playlist is a series of links to books, articles, and other materials that collectively explore an idea or set the stage for a course, discussion, or current event.
MediaWiki is a free software package originally written for Wikipedia but is now run on other projects of the non-profit Wikimedia Foundation and many other wikis. It's possibly the most popular server-based wiki software, and though it has a bit of a learning curve, many people find it to be an incredibly useful and powerful wiki tool.
MySpace is an amazing phenomenon. With over 79 Million members, MySpace is by far the most popular social networking site. (In fact, as of May 2006, it's is the fourth most popular English-language site in the world!). Chances are good that this is where a lot of your contacts hang out already.
A useful discussion of content management solutions for those interested in building sites with social networking aspects.
While not strictly about Social Networking per se, Ruby's roundup of web2 tools for social change is nonetheless a handy resource for the interested activist.
Socialtext has created a platform that blends the strengths of wikis and blogs, resulting in an interesting collaboration and knowledge-management tool.
A very popular general-purpose social networking tool.
TWiki is "a flexible, powerful, and easy to use enterprise collaboration platform and knowledge management system. It is a Structured Wiki, typically used to run a project development space, a document management system, a knowledge base, or any other groupware tool, on an intranet or on the internet."
Marshall Kirkpatrick's interview with Adam Frey (one of the founders of Wikispaces.com) about when & why wikis work best.
Wikispaces has created a web-based wiki 'platform', simplifying wikis even more. They've done for wikis what blogger and wordpress have done for blogging. They seem like great people, too - check out Marshall Kirkpatrick's interview with the founder or the 'about' page on wikispaces to see what I mean.