I think it's a feature, not a bug. It might act as a filter. I noticed that blatantly a couple of days after starting using the site, wrote this post about it: Zapping feels like playing Go!

I think the slow down in posting might be due to said filter acting, which is a very specific characteristic of SN that I actually like. SN has become my go-to network, way above twitter, because I feel I will make good use of my time. That attitude in itself is a niche attitude in the social network market, much of which is already taken by reddit.

To me a solid solution to growth is the fourth one posted here, "make every link to SN a referrer link". Simple, elegant, fair, organic.