It's just the nature of the Internet that people come and go. Some people take long breaks and then come back and some don't. I know there were a few people that joined around the same time that I did and were posting much more than me (and I know that is hard to believe
) and then just suddenly stopped. Like you, I also wondered what happened to them, but assume real life intervened. I've also noticed that people tend to post a lot less in the summer, due to vacations, outdoor activities, etc.