Even if it wasn't fad driven, there seems to be a significant trend of the number one site getting eclipsed by new up and comers. Whether that trend is coming to a halt with Facebook or not is debatable (they seem to be staying on the leading edge and more than willing to adapt and take in the ideas any competitors would push, along with the most extreme network effect possible).

On the other hand, I doubt that reddit isn't going to suffer the same fate that slashdot and digg did when something better comes along, though I doubt they are going to make as drastic a mistake as digg did.