For me it was slashdot and I let myself dive into it too much at work. I was in a very dissatisfying position at a company I loved when I started working for them but they went way downhill several years later. I was being paid but completely ignored for months at a time so I spent probably more than 70% of my day trying to get noticed on slashdot and thinking it was a viable substitute for actually accomplishing something.

When I moved on to better work I was able to leave it behind. HN is my only “social media“ now and I try to keep even it to a minimum.