i don't contribute nearly as many pageviews to hubski as i did 5+ years ago, so i probably do not represent the consensus opinion on the state of the site. that said, it is clear that some of my (and your, i think) favorite users have drifted away, despite still being friendly with various people they met through hubski.
probably for a few different reasons.
it is partially the medium's fault. interesting people just don't post on forums much. i figure that's what happened to most of the best users here; they're interesting, and therefore busy. there was a guy who posted a lot of supreme court stuff that was great. was it francopoli who had the astronomy updates? etc.
others were driven away by a few loud users who dislike people having opinions they don't agree with. some even in this thread. but that's a separate problem and certainly not unique to hubski. (although its small size makes it uniquely vulnerable.)
a couple of years ago i unfollowed all users and began screwing around with tag/domain-based use of the site to see if i could land in a bucket i find more useful, but the signal to noise ratio is still not good enough for me to justify spending much time here. (i also found that this wasn't too easy to do... domains, sure, but what's the point, i can just rss; tags, often too scattered.)
for one thing, the norm in the early days of hubski was to post the absolute most interesting stuff you found, and the average poster's age was higher than most of the internet, which meant stuff off the beaten internet path got posted. now when i skim the home page from time to time, there's a higher number of "news item" posts, i.e. ones where nothing beyond the headline is particularly relevant. there's no point clicking on those -- unlikely to breed good discussion, and no additional info in the article. that's just a reddit post. scroll and move on.
(this, at least, includes a degree of personal preference: there are a lot of subjects i find less interesting than i did in 2017 or 2014, and if those are still found interesting by the majority of the userbase, then the site is serving its purpose. but #goodlongread used to be one of the most popular tags, and i don't think it is anymore.)
so, i think the short version is basically signal to noise. i'm still pretty sure there's a way to slice and dice things so that nearly every post i see in the home feed is interesting to me, but i haven't bothered because i don't really care about the comments on any of those posts, and there are other ways to get the raw content itself.