Why not? It has all properties of a social network. Is Instagram a social network? Pinterest? Same principles, different look. It doesn't have to have "friends" to be a social network, you know.
Most of YouTube doesn't work as a social network, and most YouTube users don't use it for social networking.
I don't find the argument from features compelling because so many different services have the same features. On a broad definition, there are millions of social networking sites, but on a broad definition, the term becomes so close to meaningless that it doesn't tell you anything useful.
