I had a del.icio.us account since the beginning until it got sold, but it wasn't really a forum or hangout. It was a link aggregator.
I still have a Slashdot account, but I can't remember the last time I logged in there. Peak Slashdot was really 1998 for me, when I was working in my first IT job and would check it as much as 5x a day. By 2011, it had declined greatly in readership because of the combined effects of Twitter, Reddit, and HN.
Slashdot, though the peak days were behind it
Digg, which would be fun for a while still
Blogs like the old new thing, and Joel Spolsky
Del.icio.us and Stumbleupon and (I’ll admit it) Fark