The first three committed instant suicide, StumbleUpon and Voat by just turning off, and Digg by doing the v4 redesign that drove everyone away in short order. Technically, Digg is still around, but it's just another news site. It's kind of amusing, because Reddit was just another site until the Digg fiasco. Something would have replaced Digg eventually if they'd just left things alone, but it's interesting to speculate that Reddit may not have gotten the kick in the shorts that propelled it to where it is today. Myspace just kind of did the flounder and fade thing, a victim of corporate whipping that made it non-relevant.