What makes a social networking site different from a dating site?

Maybe this is of interest, but the first social network I remember using was actually a "dating" site, when we were in our early teens, around y2k, there were no other kinds of websites (that we knew of) that were general-purpose enough.. So everyone I knew rushed onto some site called weblove, and we used it as a general hangout, it had forums, chats, "mail" and the ability to build a profile and upload pictures. Sure, boys and girls talked, but dating was not a focus. It was actually a very nice social platform, encouraging meeting and communicating with other people in exactly the way facebook does not. Also, it was free to use.

A dating site doesn't want users to be able to associate with each other without both ends paying the matchmaker.

A social networking site wants users to connect and intermingle as much as possible so that they can extract metadata and assocations for monetisation.

Afaik you don't have an Elo rating on a social network