Delicious very explicitly did not have comments/discussion/etc in order to make it a placid place. This probably cut down the engagement, because there were no discussions to be in, etc.

I believe that most social features are really just mechanical coping mechanisms. Generally to deal with traffic, but some for spam, trolling, etc. We need to come up with some better coping mechanisms. (And news.yc has been inspiringly innovative in this regard.)