As you know a social network is a social structure made of nodes which can be individuals or organizations that are tied by one or more specific types of relations, such as values, visions, idea, financial exchange, friends, like, dislike, conflict, trade, web links, and relations or could be anything.
People have used the social network to connote complex sets of relationships between members of social systems at all scales, from interpersonal to international.
Today there are thousands of Social Networking Sites exists in internet space, But as obvious some are popular.
Here is the Popular Social Networking Sites list by forbes
Windows Live Spaces