It could be a nash equilibrium, where everyone's best interest is for everyone to switch to a new platform, but for each individual person leaving Facebook cost

It could be a nash equilibrium, where everyone's best interest is for everyone to switch to a new platform, but for each individual person leaving Facebook costs them more (since if not enough people leave there's not a big enough pool of people looking for a new social network to have another valuable social network) than the small amount of damage it inflicts on the company. If Facebook died we would all land on another platform, it's just very hard to move people over iteratively. The government could push that transition along, if it wished.