> Is that really a reliable metric for activity there?
You see, that's like naturally an indication of activity.
> Exodus
A lot of people left searching for alternatives. Well, there was none and there is none - let's face it. However some stayed on Lemmys of the world and many (No, I don't have empirical statistics on these) returned but yet didn't really return. Many returnees have a quasi-zombie-like presence on reddit now (I am one example). I barely participate and if I do it's a quip or two couple of times a month. Basically, for some reason, I just don't give two shits about the "community" aspect of Reddit anymore. No, it's not anger. Something just went missing.
I was regular at and still visit two largest of the country and city subreddits (and few more) and the decrease of quality and engagement is palpable there. If nothing - the lull and uncertainty decreased the engagement and greatly degraded the experience of being there.
Many smaller and mid sized niche subs are just gone. Either made private permanently or are left abandoned with rules in place that makes engagement impossible. Reddit can take over and force the mods out but the engagement is not coming back. Remember these places were built from before Instagram and apparently mainstream Twitter generation. I doubt that is going to happen again. It was from another time and populated by a generation that didn't mind writing long texts and finding great content and sharing without an expectation of "influencer" status and income, among other things.
Having said that, I was shocked to see some of the Lemmy instances to be really active like small flourishing islands. Check some book/book review related for example if you are interested. A friend had sent me the link once - it was some "cafe", I can't recall; there might be more. I have no interest in those instances so I never joined.
tl;dr: people who cared - a lot of them - left. I think that matters.