But users aren't locked in. They have not paid a yearly fee which still has 11 months left, they don't have to pay a fee to leave, and they don't have to pay a fee to join the other network.
Users can come and go as they please, and there are alternatives they can use, if they choose.
If they don't want to use the other network because their friends aren't on it, that's just their choice, not "lock in".
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.
Sure there is; all of the people on FB already. The fact that the software can be replicated is not the only consideration. If, in practice, they have a virtual monopoly due to user lock in then it's a reasonable debate to have.
I'm certainly out of my depth on the legal end of this, but your definition is far too simplistic.