I've been around since 2006, a while after my 7th birthday, and I've seen the numbers dwindle since then.
People grow older. Their interests change. Some people might have been involved in drama, which might have effected their mental, emotional or physical well-being - no matter what 'side' they were on (and don't brush drama off as something that can't affect people that badly, because it can, and will get rather intense) this website might have had consequences on people's school, work or social life.
Those are some reasons that I can think of. I'll agree this place is smaller than it used to be, friendship groups developed and change, likes, dislikes, the lot. But I personally think the community is nice and peaceful, and a lot better than how it used to be. (Especially for me, mental health wise)