I used to help run a few vbulletin forums, customising the code and doing admin stuff.
One of those that survive, the forum still exists (https://www.sau.com.au/forums/). It's a very specific, niche, technical forum, where owners of Nissan Skylines socialised, asked technical questions and traded.
Facebook Marketplace/eBay etc cannibalised the trading. This meant sponsors of the forum who helped pay the hosting bills slowly reduced.
Social banter moved to Facebook Groups (often more localised geographically... in its heyday in the late 90's and early 2000's the forum had people from all over the world socialising around the clock)
Which means technical Q&A is the main drawcard now, and is the most active component of the forum outside of the diehard "oldschool crew" who keep the social parts active. I expected reddit to cannibalise the technical Q&A, but it hasn't as forums present a combination of threaded conversation and blog style updates from the OP or groups of contributors.