It's made infinitely easier by being asynchronous and 99% reads. There are no timing issues. If a post is delayed to someones screen by a minute or two, nobody dies.
It's not terribly difficult to make numerous (near infinite) read-only replica's of a database which are within tens of milliseconds of the primary; so that takes care of 99% of their problems.
Handling their write load is harder but keep in mind the vast majority of their accounts are idle; and again asynchronous writes make it much much easier. They can shove everything through a message queue and put heavy-weight sharding of the data behind that.
I think handling 100 Million banking customers in 2000 was infinitely harder than Facebook has it from a technical standpoint.