I think culture is more private again. In the early social media phase, people very nonchalantly posted all their random thoughts and drinking pics, vagueposting publicly about a breakup etc. People now feel more surveilled, and feel that the public internet is icky. HR sees all etc. It's similar to how people changed their behaviors in response to ubiquitous phone cameras.
It feels paradoxical to say that, but I think it's true both that social media is bigger than ever in terms of flurry and activity, and that normal people participate less (outside scrolling the feed passively). A few people are semi-professionalizing in it now, influencers with sponsorships, local celebrities and trendsetters etc., while normal people's normal life updates are dwindling. The Pareto split is sharper.
I think that's definitely a part of it. I also think the signal to noise ratio has simply gotten so low that it's started taking up too much energy for most people, so the value is lost. There will always be "feed junkies", I suspect, but they're a dwindling minority.