Not really relevant anymore except for us old farts. I never joined FB, Twitter, or whatever. They just seem stupid to me.
It's interesting how unique each social media medium is.
Forums are nice for controlled topical discussions with people that you can build relationships (ish) with over time. Most threads are moderated & there's a smaller, recurring group of people who chill at places like AT, HF, etc.
Reddit is for crowd sourcing knowledge. Some subreddits get so much traffic that it's hard to even get any discussion going on your reply. But you get an amazing amount of data sometimes, which is awesome.
Facebook is good for IRL family, extended family, and friends.
Twitter is good for open, world-wide discussion. tbh I thought Twitter was the dumbest idea ever until I tried it, and especially for specific projects & topics, it's actually really cool. Like a massive, publicly-accessible SMS group text that goes on 24/7.
I kinda-sorta get Instagram. I mostly follow foodies. It's like...forums for people who don't want to read. And the comments are typically as bad as Youtube's are.