I agree with Lee that it does seem to be in cycles. I'm nearing the 10th year in the hobby now and I do get a little sad some days thinking about the people I don't get to meet up with anymore... That said, there seem to be a few hard-core old-timers who just refuse to give up and I do keep in touch with some of the friends I've made in the hobby even if they don't keep reeftanks anymore.
Guess life just happens and interests/focuses change, or people need breaks.
I don't think I could go for long without some kind of tank now though. I've thought of throwing in the towel a few times the last two years or so but never in terms of permanently - only in terms of "I could use a break." So sometimes you scale back, just as sometimes you scale up.
Ebbs and flows abound.
Still, I'm sorry to see people move on, and for most people, I secretly hope they'll be back one day.
Addicts need company darn it!