No one has managed to dethrone Facebook because the CFAA allows private companies like Facebook to stop anyone who'd actually provide the facilities to allow for a smooth transition. BigCos like Facebook and Google lean on the aggressive monopoly-making powers of the CFAA every day. Look up the dockets and you'll see the companies making active use of these powers.
In particular, check out Facebook v. Power Ventures, where an entrepreneur was left owing $3M in damages to Facebook for allowing users to extract only their own information.
Underpinning all digital "monopolies" is copyright law, which is eminently changeable. It really pets my peeves when people suggest things like streaming services are natural monopolies as if there is nothing that can be done about it.
Copyright was invented because since the printing press, intellectual goods are so easy to copy that it'd be impossible to sell them at scale without state-enforced supply ceilings (aka copyright).
Doesn't sound like a "natural monopoly" to me. It's wholly artificial and we could change it if we wanted.