I think you might find that the fixed cost of distributing pictures and video to 1+ billion users who check their FB hourly to be rather expensive. I think you would also find that storing all those pictures and video might be expensive. Additionally, the database requirements to search through those billions and billions of posts are not going to be small and on top of that, FB's database returns the results very quickly.
Sure, distributing the software to new servers is free. Procuring the software that needs to be on the server is another matter. This isn't a slap a load balancer on top of a few servers and a Postgres instance sort of problem.
It's not just, "I think you might find," but rather, that is precisely correct - server costs for 1 billion users is expensive. However, do you need to set up for 1 billion users in year one? Which is more expensive to start up: a railway or an internet-based software application with a very high growth rate based upon word-of-mouth?
Sure, distributing the software to new servers is free. Procuring the software that needs to be on the server is another matter. This isn't a slap a load balancer on top of a few servers and a Postgres instance sort of problem.