TechCrunch had a great overview of some top turnkey SNS providers a week or so ago. They plan to also do a review soon of some downloadable solutions and custom providers.

The three loyal readers of my blog know I had a writeup a while back of free/cheap SNS tools. phpFox, Dolphin, & Handshakes are pretty popular. Elgg is big in the education field. Pligg (and its many cousins under the Affero GPL license) is more of a Digg clone, but people keep mentioning it. And there are a bunch more.

A lot of them are written in PHP, so if you're handy with code, you can often get open source platforms to do what you want/need them to do. But remember that if you use open source software as a foundation for your startup, you may be required to release any modifications you make to that codebase. Various factors affect that, of course.

Open source exists to help people ramp up & leverage the work of a community, but comes with a stipulation that if you modify and/or try to profit from that work, you often have to release your enhancements back to the community. That's not necessarily a bad thing, as long as your business plan takes that into account. There are plenty of companies who build or leverage open source, help the community, and yet still make money doing it.