The fundamental problem is economic. It's not that Facebook is invulnerable or that it can't be disrupted—of course it can! The reputational risk Facebook is facing is existential, if it reaches a tipping point things could go south very quickly for them.
The bigger problem is that making something like Facebook requires a lot of resources. You can't achieve that scale, feature-set and UX quality without a lot of engineering manpower. Users expect these things for free, so there will always be some monetization strategy a couple steps ahead of the current cultural standard for ethics.
Startups with high-minded ideals can certainly start strong, but they will inevitably sputter since capitalist incentives overwhelm at scale. I mean just look at Google, it could be the poster-child of disappointed expectations, having come out of academia with their "don't be evil" slogan, but inevitably all large companies come under the thrall of a Wall Street mentality sooner or later. This goes triple if there is no monetization strategy and they are relying on VC funding.
I'd love to see some of these decentralized or other high-minded social media efforts succeed, but even if they overcome the fundamental technical challenges, polishing up the UX to what the masses have come to expect will require a ton of resources that will require funding that seems incredibly difficult to obtain in the current economic culture.