I think an interesting part of my experience was that it started with watching. I just loved the videos and thought they were hilarious. I admired the people who made those videos and I wanted to join that circle of creators. And Flash didn't prevent me from doing that. Not only that, but it was a specific low-effort style of animation. The path from consumer -> producer was short.
So really, I think you have to look at what people are consuming, and then try and make it easy for them to "participate." I'm not sure if people are consuming Flash-style animations anymore. (Maybe they are, and I just aged out of the demographic.)
It seems like a lot of that creative energy has moved to sandbox games like Minecraft for now, where that community of creators and would-be creators still exists, and away from animation and Web design.