Categorization of social networks

I had an idea in the area of social networking. There’s not much to say about the idea yet, but I tried to figure out a scheme that I can use while analyzing other social networks. There must be plenty of categorization schemes available on the web, but I tried to focus on the main use case, the “added-value” to the life of a registered user of such a social network. I came up with the following short list, which was sufficient for me to categorize several dozens of social networks

• Knowledge Sharing
• Connect with existing friends
• Find new friends
• Find your better half
• Present yourself (& your friends)
• Share my lifestyle with others
• Extend Business Opportunities
• Indulge in reminiscences
• Enrich personal life

Of course, many of the social networks I analyzed belong to two or even more of these categories, e.g. there are many special intereset groups that would fit “Share my lifestyle with others” and “Find new friends”. Do you think there’s anything missing on this list?