That really doesn't seem like that hard of a thing to answer. E.g. just look at Facebook: name the #2 broadly "generic" social network. Is there really any other alternative here, at all? It really was a winner take all effect here, evidenced by the fact that all their former competitors are basically dead. Same thing with Instagram - anyone remember Hipstamatic?

Twitter and Snapchat are the obvious answers here about “competitor”.

And WhatsApp, of course, the competitors are iMessage, GChat, and MMS/RCS that’s already built right into every phone.

What is a social network? What makes Facebook different from Twitter, Reddit, HackerNews, or Linkedin?

Basically if you define social network as something exactly like Facebook then, of course, that's what it is and everything else is excluded.