I think Reddit also kind of replaced it. Back then, Reddit was more of a social bookmarking site like del.ic.ious but with expanded community features. You would go to the front page, and see a pretty similar selection of links to what you would get smashing the Stumble button that day, only it's been curated and sorted by the community through the karma system. Each link also came with a very well designed comment section where users could discuss it.
Correct, it was a mix of Digg, Slashdot and old-school message boards. As far as I remember, it was the comment section that most people were interested in. The link itself came second.
