Could you use that data in a logarithmic scale? I believe It would give a better understanding of what all that stuff is on the bottom, because right now it barely registers as noise.
I also think an explanation of the more "constant" second jump can be attributed to the people that visited the first time actually giving the site a second shot and getting to understand the inner workings of hubski.
I'm one of those people. The first time was more of a visit and forget because I couldn't find the topics I wanted to, but now, with a second attempt to use the site, I made an account and really delved into what the site is, and I am really enjoying it.