From my perspective, the RSS reader mostly killed Slashdot for me; Slashdot was great when it was able to surface things I normally wouldn't have seen otherwise. Once RSS (then Twitter) came along, the sites that were generating their own content were easier to find/skim directly rather than waiting for Slashdot to throw them a little publicity.
I frequent HN because it does serve that purpose that Slashdot once did - bringing content to my attention that I otherwise would have missed.