That doesn't explain salary discrepancies between the US and the UK at the same company
A Facebook engineer in London is probably earning 70% of what a Facebook engineer in the US is earning, even after taking in to account taxes, benefits and any cost of living discrepancies.
This is driven by salary sharing companies, such that companies benchmark to each other, which tends to be lower.
For the specific case of London (and more specifically Facebook), this is driven by the fact that FB/Goog don't count tech people in finance as appropriate comparators. I suspect that this is not true in NY/USA.
A Facebook engineer in London is probably earning 70% of what a Facebook engineer in the US is earning, even after taking in to account taxes, benefits and any cost of living discrepancies.