[et-al]

I think it's because a lot of those giants were commodities and could easily be replaced. Companies have learned the impact of branding, and they're now tied into our identities. So the social aspect keeps companies relevant and in power.

With regards to Facebook, when I'm looking over people's shoulders, what are they doing? Checking out their Facebook timeline, or Instagram, (or playing Pokemon Go for the time being). To unseat Facebook, Facebook would need to really betray the public's trust, and a competitor would have to provide an equally comprehensive service (and helpful migration tools). Google Plus's timing was poor and seemed to share everything with the pubic. ello had good timing, but they were invite-based and the minimal interface seemed like it lacked features.

Facebook is the AOL of the present, but unlike AOL they've managed to adapt (or acquire) to users' changing mediums for socializing.