TechCrunch has published a comparison chart of 34 social networking companies and their products. It's not the large colonies of MySpace, Facebook, Bebo or LinkedIn, but the products/services aimed at enabling businesses with social networking software. The chart allows you compare a subset of products, and the data, mostly from the vendors, is comprehensive. It helps if you have a large screen.
One of the commenters, Interactive Visionary, on the post offered some good advice for companies looking for a 'white box' social network:
...read the fine print and do a deep dive on hosting, site maintenance, page views serving, number of members supported, cost of added components like video upload and streaming, etc. before selecting a technology partner. What may at first seem like a bargain may be costlier than what you first saw as being expensive.