The former is inevitable - most people, like the author of this article, spend the bulk of their time up-front: post a lot of answers, enjoy the experience, get tired of it, pull back to only answering the occasional post that piques their interest. So there's a fairly constant fall-off of activity from people who've previously posted a lot. And this gets worse when these folks find fewer questions that interest them, which brings us to...
The number of questions posted on Stack Overflow every day is overwhelming. Over the past year, it's ranged from a low of about 5.5 thousand per day (Christmas holidays) to over 11 thousand per day; currently it's averaging around 8.5K. Depending on your interests, you might see very little of this (if you focus solely on a relatively obscure tag) or all of it (if, as many do, you drink from the firehose at http://stackoverflow.com/questions/?sort=newest) or some mixture (if you stick to the home page).
The folks who stick it out long-term tend to have some pretty gnarly filters in place. Those who don't tend to become very bitter about the whole Eternal September thing, because that's most of what they see. The chance of dropping in on Stack Overflow over lunch & just stumbling upon a new question that covers some novel, interesting tech you've been working with lately is pretty slim, so if that's all you're answering anymore, well... You're probably not answering very much anymore.
The trick here is to find a way of putting more interesting questions in front of the folks they're likely to interest, and showing fewer "here's my homework, verbatim" questions to... well, everyone. And it's a hard trick.
Disclaimer: I work for Stack Exchange and have been hanging around Stack Overflow pretty much forever.