I think a lot of this is just the changing nature of the internet. Forums in general have far less traffic today than they did 5-10 years ago. Social media is still a relatively new thing, which is easy to forget. As far as the full-on social media storm that has taken over the internet, I think it's really mostly within the last 5-6 years.
There are a TON of Fallout groups on Facebook that get a ton of traffic. I'm in several that have 10,000+ people with tons of activity. They definitely have more traffic than the forums here do. However, the quality of here vs. there can't even really be compared. I post a thought provoking topic pertaining to the game or a strategy to accomplish something in a Facebook group and it'll get a handful of likes and comments. I post it here and it can result in a long and further thought-provoking discussion. If I were to post a stupid meme about Preston Garvey on a Facebook group, it'd get 100+ likes and tons of comments...whereas if I were to post it here (I wouldn't anyway) it would get a handful of comments and several people telling me that it's basically a waste of bandwidth.
The quality of the discussion here (and on forums in general) tends to be much higher, but the quantity has gone over to social media. I can't even tell you how many times I've seen pictures posted with the caption "finally got a full suit of X-01" with yet another screenshot of the suit of X-01 on the 35 Court roof. I've seen it hundreds of times...and yet people still feel compelled to share it in the Fallout groups as if it's some kind of unique or difficult accomplishment. This is the case for basically any topic. I'm a big fan of The Walking Dead. I'm on the official Walking Dead forums, and I'm in a number of Walking Dead groups on Facebook. In the Facebook groups the comments are brief, lacking much in the way of real thought, and pretty basic in the thoughtfulness of their execution. On the forums, people will post detailed anolyses and reviews of the episodes, including potential interpretations of symbolism that can provoke a lot of thought and discussion, detailed anolyses of the motivations of characters, predictions for future plot points indicated by elements within episodes and so on.
Social media seems to inspire stupid questions. I can't count the number of times I've seen incredibly simple questions like "Where to buy shipments of wood?" posted verbatim just like that. I mean...come on. For starters, you could literally just type that into Google to get your answer. Putting that aside, at least form a complete thought. Give me something to read. Something like..."I'm building at x settlement and I keep running out of wood in the middle of my project. Where can I find a good supply of wood? Which vendors sell shipments of wood?" At least the second version contains some kind of thought and carries some kind of potential for conversation. Answering the first question usually brings a response like 'thx bro' and that's it. Granted I'll see an occasional thread posted here with a similarly simplistic form, but it's far less common here. Questions like that are a constant thing in Facebook groups.
I grew up in the 90s and early 00s, which was really the heyday of the forum format. I'll always prefer them, and that's not likely to change. But these days you basically need to have a blend of both.