The thing is that is the employees that pay the the income tax _not_ Facebook. Facebook (and others) are still not paying the taxes on the the income they earn from operating in the UK no matter how many people they employ at high salaries.

The choice you present is a false one, the multiplier productivity effect that Facebook employees gain from operating in London as opposed to Dublin far outweighs what Facebook would end up paying in corporation tax under a more reasonable system.