Social networking, like a lot of other business segments, is starting to encompass lots of other companies doing similar things. Increasingly, social networking is about finding people with similar interests, backgrounds, or personal attributes (race, religion, disease/disability) and getting them to talk/share stuff with each other. What's driving startups is that they're going after specific groups/markets in the hope of getting enough of them together so they click on an ad every once and a while on the web site.
The best example I can think of that goes after a specific group is Dogster. Dogster was started as a joke by someone who created the site over a weekend (at least that's the story I heard). Much to the designer's surprise, people really wanted to talk with other people about their dogs. Dogster has since raised some money and is getting revenue from sponsorships. Woof!
Increasingly, people/groups will just put these networks together themselves instead of relying on a business to do it for them. Ning has been working hard and has a bunch of social networks already up and running. They make it easy for people to put together all the components (forums, photo albums, video, etc.).
Plug: Mobile Monday Boston will be having its next meeting on Sept. 17 to discuss mobile social networking. A number of companies will be presenting (including mine - padpaw.net).