I don't believe it's a zero-sum game between forums and social media. Yes, it certainly isn't like the old days where you can build it and they will come. Very different today.
However, I believe that in most niches, social media and forums can co-exist and complement each other. For example, my niche is freshwater planted aquariums. Most of the activity has moved to Facebook Groups over the years. I started two months ago focusing on the needs of that community that social media can't fulfill. We've been growing. Slowly but that's OK.
On my site, a member can create a diary of their aquarium and have people see it, offer help, support, etc. People can find longer articles on subjects and participate in photo contests, etc. None of these things work well on Facebook where you really can't post more than 2 -3 lines into the constant waterfall of messages. If you get one or two valuable responses, you feel good.
I encourage my members to use both as intended. Longer talks, relationship building, diaries, contests, etc should be on forums. When they just want to go to a less thinking more eye candy environment, then Facebook and Instagram are there for them.