I have had arguments with a lot of buy side and MBA types who try to convince me that buybacks are good for the country and patriotic but I would prefer a dividend by god. buybacks to me are un-American especially those funded by debt.

And the market is free to consider this an unwise investment and tank the stock. ¯\_(ツ)_/¯

Also, since FB is a publicly traded company, it's not actually "his money". It's the shareholders' money (of which he is but one), and thus he has a fiduciary duty to act in the best interest of those shareholders.

https://www.law.cornell.edu/wex/fiduciary_duty