I've been a regular
in various Usenet
groups for some years now, and guess what
? Somebody asks that question
about every 6 months, with startling regularity
And they always seem to be particularly worried by the declining percentage of people who've been there longer than themselves - each generation has its own Old Farts.
People have lives. They get bored with stuff, or they graduate from college, or they move jobs or have kids or whatever. Real life will intrude.
And there's always someone doing a bit of a public whine and leaving in a huff. Good riddance. Believe me, I've seen it all before, and the group always survives. It's bigger than the sum of its members.