@baishuu "active users" can mean a lot of different things. I can say that there were 549 posts in the past month. The numbers I have for "unique visitors" and "page views" are so high that I suspect my data doesn't properly differentiate between human lurkers and bots. I definitely can't provide user trend data because we just don't have historical data on this stuff for the site.
Anecdotally, I can say that I can never remember a time when people haven't been saying "TMD used to be more active", even 15 years ago when the site was brand new and people were saying "Beyond Dominia used to be more active."
I don't have hard data but I do have a working theory. It is extremely common to see people go through phases with this hobby (like any hobby, of course).
People start out excited and passionate and consume every bit of information they can find. At this point a player is fascinated just reading lists of archetypes and historical results. Maybe they feverishly hunt down fundamental general theory pieces like "Who's the Beatdown?" Maybe they buy a physical copy of "Playing to Win." Thinking about the game in a new way, each win is thrilling and motivates them to learn more, each loss is a challenge to find that piece of the puzzle they're missing.
At some point they find a set of strategies that work for them and they start specializing. They don't really need general information anymore, they get the fundamentals, they want to learn what successful players are doing and replicate that. These players hunt down tournament reports and sideboard guides and archetype megathreads.
Some players have a knack for the game or a lucky streak and start picking up wins and the minor notoriety that comes with it. Notoriety is fun (speaking from experience) and these players tend to get excited about putting their own mark on the metagame. They want to take a crack at writing their own primers and tournament reports. A player with a lot of hubris, like my younger self, might even try adding to the great canon of magic theory.
Whether they have tournament success or not, eventually a player hits a plateau in their results and their interest starts to wane. Maybe they still like talking about vintage, but at some point reading primers and sideboard guides stops making them any better. Maybe they think their own techniques are better, or maybe they just aren't getting results from what other people write. Either way isn't making them any better at the game, so motivation to read them is low. On the other hand, "vintage issues" topics like WotC announcements and restrictions don't suffer this problem (they were never supposed to make you better). Proportionately, more of their conversation will be about these subjects. Some may refocus their efforts on community building, maybe through TO'ing or Judging or facilitating content creators.
Interest waxes and wanes, and maybe not everyone hits all the notes in the same order, but I think most players do. Some of the excitement can't be recycled, either. New cards can be printed and new strategies learned, but you can only read "The Danger of Cool Things" for the first time once ... even though people keep writing it over and over again. When you ask someone about the historical health of the vintage community, it's probably not hard to listen to their answer and pinpoint exactly when they started playing.
I think "the downturn" in the hobby as a whole (which includes TMD but certainly Facebook and many other avenues as well), is a lot like an adult watching a child's cartoon and saying, "when I was a kid these were much more entertaining." The cartoons didn't change, you did.