If it costs N$ to purchase one developer in Kansas, and $2N to purchase on developer in SF, if there are enough developers in Kansas to satisfy the demand, then yes the price drops to $N. But if in fact the demand is higher than the supply Kansas can provide, then Facebook will buy out Kansas, and get its remaining supply from SF.
The average price of developers goes down, yes, but you get a form of price discrimination since those in SF value the city at more than the increase in relative salary from leaving.
The Kansas developers could increase their cost to compensate, sure, but you run into a few issues, while the market is supply-limited otherwise this wouldn't be an issue, if Kansas devs cost the same as SF devs, FB might just revert to its old practice and leave the Kansas devs back jobless.
You can sort of think of this as high CoL cities having a monopoly effect on pricing. "Normally" the market would fix this, but people are attracted to cities for lifestyle perks that can't be made up with equivalent $$. In other words, the city has an absolute advantage in city-lifestyle, so others can't compete on price to people who want city-lifestyle. In other words, for the person who enjoys living in SF, you'd have to offer them more to work in Kansas than to work in SF. As long as enough of those people exist, the market won't equalize. The normal econ-101 kind of understanding of supply and demand doesn't work, because you have, in essence, one consumer who wishes to purchase from two different markets with different market prices. City-livers and non-city-livers.