Facebook avoids taxation just like Google, Apple and countless other multi-national corporations. You don't even have to be in the Unicorn club to make entirely legal tax avoidance schemes a worth while endeavor. Call up EY, PCW, or really any law firm/accountant/tax advisor and you too can enjoy an effective tax rate in the low teens or even lower.

They do pay taxes. They play loopholes in numerous jurisdictions to avoid tax as much as possible.

Why do they do that? Because everyone else does. Also they have a fiduciary duty to do so. Yes, it's technically, actually, illegal for them _not_ to avoid taxation as much as possible because, well, then they wouldn't be doing everything they can to maximize shareholder value.