I only joined here a few months ago, but even I'm see an awful lot of repetition at times. This, I suppose, is quite natural as more people discover self-publishing and have the same newbie questions we all had at some point.
Having said that, I still find this place a great resource, and I'm constantly finding new information and opportunities, not to mention helpful tips and tricks. And because of all that I've learned here from so many kind and generous people who took the time to help others, I like to stick around and give back wherever I'm able.
As for people disappearing, that's also quite natural. Some, I'm sure, have become so successful that they no longer need to spend time here anymore. But for others it may have been more of a fad, so they'll have uploaded that novel they wrote five years ago, got a few sales, enjoyed the experience, and moved on to something else. And this, I believe, is a trend that will continue - a lot of people trying it out but not in it for the long haul. After all, as wonderful, simple and easy as self-publishing on Amazon is, it still takes a lot of time and commitment in following through with marketing your work if its to rise from the pack. A lot of people aren't going to want to do the grunt work that goes along with it.
One other thing about these boards, as someone mentioned, is that they can be addictive and consequently a huge drain on time that you should be spending on your next book or WIP. So, like most things in life, a healthy balance will do you the most good.