Facebook has x billion users. What you do have net 0% impact on them, but a large impact on you. So people block ads and use Facebook anyway.

But a more direct answer to the question is they don't think about it, they don't care about it, or they don't want Facebook to stay in business. If Facebook crashed tomorrow I would need to start emailing a bunch of people to put together contacts but other than that it would be a net benefit.