VMS3.info looks at the major social networking sites via the Value Framework.
This analysis examines nine firms in the industry, exploring their similarities and differences. Each firm brings a new perspective or feature to the mix. The users, members, and customers decide which are valuable and which are not.
The actual analysis is quite "light", but it was interesting to see 22 different companies identified as "social networking sites". I never really considered Classmates.com to be one, but really it is -- it doesn't matter that I don't like their business model and haven't found any useful features...
Zerendipity wasn't included, but that's likely because of its (to date) exclusivity and geographic base/bias in Vancouver.