There already is a financial incentive to misrepresent location regardless of whether Facebook is paying the same by location or not: locations themselves charge different tax rates, and there is a big temptation to say you live in an income-tax-free state and locality even if you do not. So this compliance problem exists regardless.

Facebook has a big target on its back here, and it would be terrible press and regulatory scrutiny if it came out that a large number of employees were evading state and local taxes by claiming false residence. Facebook can do some basic due diligence here to prevent that from happening.