Some social media may be used like that, but that's not what social media is. Message boards are an older iteration of social media (before the term gained popularity). Make no mistake, when you post on a messageboard, you are engaging with social media.
indeed. Any programmer worth their salt can reproduce the basics of MySpace/Facebook/LinkedIn/NextDoor which is posting and an activity feed/wall.
All that data could be mined, or not. That's independent of what the users are doing, which is flagging other users to be included or excluded from their view (friend/block) and posting stuff.
Forums are really the same concept, except it is post oriented (I start a topic post, you reply), and the Facebook-like systems are people centric. It's still the same mechanics in the database, just a difference in the emphasis in the UI.
What's evil about it is simply whether the host is using your data.