As a heavy forum user (since the late 90s), I find reddit supplanted them. Reddit is basically every forum I've ever been a part of, all on a single website. More importantly, it's every forum I didn't even know existed, on a single website. I found many niche hobbies and interests I wouldn't have otherwise found thanks to reddit.

I think that's why reddit grew so large, so quick. It does what forums do (provides similar discussions), except better.