I would see it as a Forum because of all the special groups you can join and debate or discuss things. Most Q&A sites will do their best to stop people from "hooking up" as you say. That's what private messages are for. This, being a Forum format makes it a Q&A and a Social media site. Most of the better ones are now.
One could say that a "Q&A" site is a variety of social media site. I do tend to think of vapid sites like Tumblr, Vine, and Twitter as "social media", but that doesn't mean this couldn't count as such as well. It's a site intended primarily for interaction between users, so in that sense, it is "social".
Personally I come to a Q&A site to debate... discuss.. learn.. and inform.. Dating sites are for hooking up and social media is for being light and fluffy.. there's probably no hard and fast rules but general expectations.
