There are several sources of information if you are looking for a list of Social Networking Sites. Rand over at seomoz even out together a web 2.0 list where they rated the best sites in each category.
The community over at Wikipedia have also compiled a very useful list of social networking sites. The Wiki list includes information about the focus of the sites and user size.
Either list is a good starting point if you are researching social networking sites that are in your vertical market. Before jumping in, I would recommend observing the conversation first to make sure that it is a neighborhood that you want to be in and then join in the conversation.
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