Gaia fits into a very specific niche of anime-inspired forums. The avatar customization features help to maintain its user base, but the truth of the matter is that Gaia's theme is very geared toward younger people. There's nothing wrong with that, it's just the niche that Gaia fills. It sits very firmly in the early 2000s high school brand of teenage indulgence, which is lucky for the site since that's rather marketable.
But the reality is that this niche is something most people grow out of. Most people I know tend to look back on their anime-obsessed days of lolrandom identification with grim embarrassment, preferring to simply ignore those bygone years of verbalizing ~nya at the end of their sentences, or forming a highly Japanese nickname, with full honorifics, and being so proud of that. Not that everyone on Gaia is drawn to the site for that reason, but it tends to be the general glue that holds most of the place together.
You really can't go anywhere on this site without coming across at least one person who earnestly call themself random, touting it as a high form of comedy, and who will proudly inform you of the cat-themed accessories they always wear.
Gaia caters to immaturity, and not the kind of immaturity that everybody with a dissenting opinion is accused of, but genuine immaturity. There's nothing wrong with that, of course. We learn maturity through immaturity. Immaturity is the stage in our lives where we experiment to find out who we are as people. It's an important part of every person's life.
Interests change, however, and many people find they aren't that drawn toward the general style that Gaia presents. The friends they've made have usually moved on to other websites, and so the appeal of Gaia wears thin, to the point where people just stop going.
For most people it really isn't a dramatic exit, but more like Gaia takes the very backseat in terms of online social obligations. You know what they say, those who leave in dramatics are often back within a week. So it goes.
For me, Gaia really just doesn't have anything for me. I get on when I wan to pass the time, but I really just don't do anything. I don't visit any threads, there's nothing really to discuss. I don't RP as much as I used to, not that the high school angel/demon/naruto variants that bloat the forum have ever appealed to me. I don't post art or writing, the interface can be frustrating. The only thing I still do, and it's been the one constant, has been lurking the GCD. I used to be active, but I've stopped being such an indignant little s**t, so really there's no reason to make threads about the economical crisis that is the Gaia marketplace, or trying to find out what the current thing is that's slowly killing Gaia.
All for the best, really.
The GCD used to be incredibly pretentious, which might surprise some people who weren't around as little as six years ago. It's pretty interesting seeing how much has changed. I'm rambling.