I've seen it happens lot over the years, here and at KF, where I'd been a member for many years prior to starting Multitool.org and it's sad as I miss many of the people, but there are a lot of factors that affect it. Much of what causes people to go away and not come back is a change of habits.
If someone gets distracted for a while and then get out of the habit of checking in here regularly then they often won't start again until or unless something comes along to change that- maybe a FB post, maybe they have a qurstion, maybe they see something that reminds them of the forum or another member somehow. Without a reason to come back, many won't even think about it. It feels a little weird to suggest that this forum, members and the sense of community we have here can be so easily forgotten, but the reality is that when people's lives change it often sweeps a lot more away than you realize.
Def