No. I'm sure plenty of the old users are still there. The problem is that there are too many new users polluting the community with less than worthless comments and submissions that are completely off topic from what was once the norm and driving force behind the community.
Reddit was once a great place, but its time came and went. Now there's n.yc, and whenever this site starts to drift off on a tangent, something else will crop up to keep us entertained. I think it's just the cycle of life for social bookmarking sites.
I have been working on an algorithm to solve the problem, but I'm not yet ready to release it out in the open. :-)
I also found the approach to solving this problem by the Jaanix website fascinating (http://jaanix.com/).