One type of “social media” is actually about the oldest public use for the Internet: Forums (or as they were and still sometimes known, Bulletin Boards).
Of course like everything else they’ve grown up and can now support almost any functionality you would want. Probably the two most widely used platforms (programs) are phpBB and vBulletin.
The basic idea of a forum is a way for a group to interact in a selective manner amongst themselves.
The content is in “threads” much like email, except instead of getting bombarded with an email every time someone in the group adds a posting, you can go to the forum whenever you want, or not, search or browse what you are interested in. You can also set it up to notify you by email on a selective basis.
A forum can be open (anyone can see content, join, and add to the forum), partially open (anyone can see content), or closed (you can only see content if you are a member and only join on approval).
Forums are extremely useful in specific situations. One is if you sell software, or anything for which you have a users group, a closed forum for the users to interact and build a treasure chest of questions and answers about the product or service. This can build up a knowledge base far more complete than staff could ever create, and with far less work on their part. And users can really get into this (much like some people live to post on Wikipedia).
Staff can also answer questions and use the forum to inform users. It can be a way for users to request features, and it can be a way for management to see what the clientele think of the company, its services, new offerings, and so on.
There are many other ways forums can be used. People into Greek cooking trading recipes. People who are into online vampire games. And so on.
If you can conceive of a situation involving online interaction within a group, probably a forum is the answer.