It seems like you've identified a problem that Hubski hasn't grown much in its 10 year lifespan. My experience is that there is never enough content (posts & comments) on the site each day. I'm not sure how showing users even less content (comments from new users) is supposed to address this.
The digital marketing growth hacker types like to image a funnel that tracks a user in their journey towards a desired endpoint. Some percentage of users drop off or fail to stick around at each level of the funnel. This makes the higher levels of the funnel the most important places to influence behavior, because you have more users to work with. The experiment here seems to be fiddling with the Retention of users who have already found hubski and registered an account in the Acquisition and Activation levels.
I have no insight into this site's actual traffic numbers or how many new visitors check it out, but that is where the focus should be to grow this community. More potential friends need to be browsing the front page so they can be compelled to register some day. It's hard to see names that we know and recognize leave the site, but attrition is natural. Without a consistent source of new members, this site will inevitably shrink into a devoted core. At that point there probably isn't critical mass to keep a site like this alive, and the remaining few will probably transition to a group chat or other private discussion to keep in touch.
I love this site because of the serendipitous nature of the content and people here. Without that new energy, I may as well just subscribe to the kleinbl00 newsletter and move on.