If you think about it, any platform that possesses the function of sharing, rating, or reviewing content coupled with a medium to chat or "create a network" among different users with a profile through a comment system can be viewed as a social network. This is because I think it is the criteria that all existing social networks have. For instance, technically, Blogger is a social network since it fits that criteria.
I suppose a site like this would have to outright call itself a "social network" to be viewed as one. When it comes to Youtube, its primary function is having its users create and share videos. In that regard, it's just a "different type" of social network.