I've noticed this myself and, sadly, I'm one of those missing from the site. I notice that my last posting was on October 10 and still appears in the "new Books" listing. I remember not too long ago when any given post would be knocked off the list in a short as time as a week or less. I've been on the site for almost fifteen years. When I first joined, there were a great deal more active members. Following the Big Crash, when we lost two years of posts, that seemed to turn off a lot of members who never came back.
I used to do a large amount of reviews most every time I logged into the site. Now, I hardly do any. In my case, I'm sure my sloth is due to having amassed almost 1000 points. If I want to post anything, I already have the points I need, thus there isn't a push to gather more by reviewing. I know this is a psychological reason, but it still exists.
I've been procrastinating on my current novel, What Doesn't Kill Us..., for some time now. My brain says "Why doesn't anyone review it?" and then my more practical side answers by saying: "Well, you didn't review them so how can you even wonder why they don't review you?" It's a lose-lose situation and I. for one, am having a hard time resolving it. Even my writing/editing here at home is affected by this procrastination, and I feel badly because of it.
Bill