You're right that the analogy was not quite right.
Here's a better one. Facebook creates a virtual world where 50% of the population choose. Apple creates a different virtual world which the other 50% choose.
If you simply look at the boundary/entrance you can say there's freedom of choice. But whether you choose apple or Facebook, there's 0 competition once you're inside that world.
Further, once you've established a home and connections within one world, switching to the other becomes quite expensive.
Competition at the gates, monopoly within.
I might add that this is very similar to the concept of company scrip whereby the local coal mine had a monopoly over local jobs and gouged workers for basic necessities. Absolutely you could have moved a town over, but the cost to do so was deemed too high in many cases.
When cost of switching is high, monopolistic power can be enforced upon customers. It's a similar situation with a lot of SaaS who have pricing power to strong arm their customers into high margins due to the cost of switching to alternative technologies. In a truly competitive market, SaaS margins should be close to 0. Obviously that's not the case today.
Regulation will catch up to all of these tricks, just a question of the timeline. Capitalism only works well when there's an environment of competition, and leveraging high costs of switching or large gated systems to enable profit margins well beyond what a competitive market would bear is antithetical to this concept.
That's why antitrust law is so important, and needs to evolve to handle modern business structures.