It’s quite a way back now, so my memory is a bit fuzzy! I think there were redesigns and the site was sold to a company who mismanaged a few elements. I seem to remember a lot of noise on the site at the time, people complaining etc and then a large number moved away. I’m sure someone can fill in the details in a more reliable way haha.
I remember Digg used to be great too - between that and Stumble Upon you could find so much awesome stuff!
I suspect Reddit will die a much slower death, as it is just so big and widely used and the exodus of users doesn’t feel as huge. It will be interesting to see how everything unfolds.