Although it refers to a specific moment in history (http://en.wikipedia.org/wiki/Eternal_September), it’s something that can happen to any community at any time. The problem isn’t just that the original community is overwhelmed by the massive wave of newcomers and unable to “educate” them, it’s also that the newcomers don’t feel a need to learn the rules when they first show up.

Some thoughtful comments regarding an online community maintaining it’s culture and quality as it grows: http://paulgraham.com/hackernews.html