Well I can say I see a lot of people on my IM and in IRC that don't post much anymore. But yes there seems to always be new blood. Honestly sometimes when you've been on a board for a long time you see the same questions over and over and over and ... well you get the point. It get's kind of tiresome answering the same thing again and again. You have to admit even Jon doesn't post that much anymore. But a more telling number is compete as I find it a lot more accurate. Here's a comparison of wf, dp, and sp.
SnapShot of sitepoint.com (rank #4,240), digitalpoint.com (#2,511), wickedfire.com (#28,734) - Compete