None of this is new. I've been running forums for well over 20 years (before @Matt was born 😉 ) and so I've seen this debated multiple times over the years.
I am also an admin and moderator on some quite large facebook groups - despite some of them having 25,000 plus members they are slowing down big time. I now use them purely for business purposes & holiday snaps and nothing more. There is no organisation with social media - Facebook is trying it with tags, but very few people use them.
What is harder is for new forums to break ground and be a large success. I've seen forums attempted to draw in people off the back of very large facebook groups and they have failed. Maybe it works better in some fields, but at the end of the day the forum will only work well if it has a good website behind it, not as a stand alone. Of course there will always be exceptions. But content is always king.
What is needed is good interaction so that it is easy to use all of the social media platforms in conjunction with a forum. Post on a forum and be able to instantly have your posts show up on all the social media for example.
Forums will never die - sure brands might come and go, and I've seen a few over the years, but they will be there for as long as the internet is.
As long as they can keep up with modern technology, there is a future.
They will outlast me, that is for sure.