I was a long time user too and I even moderated a few small subs and I was active in the groups I was with. I was a user for ten years and I grew these groups I worked on. After the change I gave up all four of the communities I ran, deleted my account and never looked back.

I think the explosion of popularity came as a result of the API change fiasco and the protests that people created. Reddit became headline news all summer and I think new users flocked to it because of that. The problem is that most people don't care about creating content, they move over to find content.

Like everyone already said ... The Reddit change brought in new lurkers that only want to watch while at the same time most of the popular creators left. There are not that many popular creators or active users who like connecting people because it takes a lot of time and work to do .... for sure it literally becomes a full time job. When a website loses those core people, the content changes and becomes less interesting.

I go on Reddit once in a while to check in its status and if you notice, a lot of the popular subs have slightly decreased in activity but if you look at the forums, a lot of the content and activity is recycled from years ago. Reddit can probably live on recycled content for years but it will be a decline and the decline will take a long time before it becomes obvious.