But they still READ. So, if you 'interact' (and by that I mean do any write-like action, like commenting, posting, liking, whatever) less, that's gonna show up.
I think a decent-sized subset of Millennials have basically aged out of the time-surplus years of the early 20s and are now busy with kids and careers and families. And they aren't being replaced by the new 20-somethings, at least not on social media of the same form. The kids are still on text messages and Whatsapp and Discord and Roblox and Google Docs (!!), but they aren't interested in getting on the public Internet, and if they are, their parents won't let them.