I used to run a fairly large social networking site (before it was called a social network) with forums, photo galleries etc. and had a similar experience. We changed the design fairly drastically and there was mass revolt.

Two competing sites were started that were similar to our previous design. For weeks it was all anyone talked about. People left, they said they'd never come back and they joined the other sites.

After a month or two people started coming back. They started to see the merits of the new design and features. After 3 or 4 months it was if nothing had ever happened. The competing sites fizzled out and most of our users returned.

Lesson learned was that people hate change but if it is change for the better they'll generally recover from it.