Mark hasn't been improving the Internet so far (actually FB made it worse in numerous ways), so why do you think he has any incentive to improve the world as a whole ?

I'd say you're making the assumption that it will be used for good, when so many charities are indeed used for tax evasion (and other more nefarious purposes). When you condition these probabilities on how most charities actually operate, and on Zuckerberg's past behaviour, a high level of cynicism proves to be the rational position.

Facebook is abusive of people's personal privacy, even though it is generally an effective tool for improving people's (layperson's?) communication.

So, it's probably a better idea to approach it with a cynical perspective instead of an optimistic (naive?) one. It could turn out to not be estate-tax evasion... however it seems unlikely.