Where did Facebook say that "cost of living" will impact salaries? At Google (where I work) salaries vary a lot based on location but cost of living is not part of the equation. It's based on the cost of hiring in the market.

I'm personally in the middle of an SF to London transfer. I'm taking a 25% pay cut. That is not because it's cheap to live in London (it's not) but rather because it's very very cheap to hire developers there.

I imagine Facebook will do something similar.