Your premise only exists in a simulation where humans aren't human, where instead they're robots that think solely rationally and are not inconvenienced by anything. That's nearly the exact opposite of how people behave.

When you're actually dealing with human customers and users, it's nearly impossible to convince them to switch once they're comfortable with a product or service. Those people have to be tremendously inconvenienced to do so, while simultaneously being presented with a great alternative - often it requires a 10x-style superior product that isn't just great, it's a huge leap over what's already available.

Switching costs are immense with Facebook. If you have 100 friends on there, and you're the first to abandon their network and move to a new social network where none of your friends exists - that's a huge cost for the average person. It may or may not be a cost to you, it is an immense cost to most people. That also applies if you attempt to maintain profiles on both networks, it's a time and annoyance/inconvenience cost, people hate such.