First online social network – Indeed it is a matter of definition, and I would give joint honors to Six Degrees and also the earliest of the BBS systems.
A social network is one way of describing a system with multiple entities (typically humans) who interact with each other socially. In that sense the first online social network happened the first time three or more people interacted among themselves online, indeed on a BBS in the early 1970s or even possibly on a standalone device with multiple terminals or accounts. All kinds of things create online social networks – email, MUDs, trading marketplaces, document collaboration systems, even telephones in a sense.
One big innovation in online communities, which social scientists had hit on decades before, was to map out groups of people not as a list of members (i.e. groups or tribes), not as a list of nodes with different connections (a contact list), and not as a bunch of people with different profile attributes, but as an interlinked structure defined by the connections between individual people rather than the attributes of the people themselves.
Six Degrees was not the first online service to do this, but it was the first to self-organize by allowing existing members to invite other people as friends, and then to create the links by accepting or declining friend invitations. Before that the network was either intrinsic (you could infer it from each person’s lists of contacts, but it was not formally structured as a network) or constructed (a scientists would watch a bunch of children play and draw out the social graph based on their lab observations, rather than letting the children create it themselves).
Most social networking services are not pure social networks. They include groups, tribes, user profiles, and lots of other add-ons, basically, whatever works. Friendster is a social network, sure, but it’s also a great big data archive, content system, application platform, and lots of other things too.