I'd rather see them pay a considerable chunk of those billions in taxes - which Facebook and other giant companies like it don't pay in the US due to its lax tax laws, and which they evade in other countries using various tax evasion schemes. Then there'd be more money to pay for affordable schools and whatnot.

Then again, the US is more likely to spend trillions on wars abroad without those really improving their own country, so on that note, it's probably better for a philanthropist to invest the money in a charity with a singular goal, such as education or research.