His company, Facebook, is infamous for not paying taxes. (On profits of three billion last year, it paid six thousand dollars in corporate taxes.)
Now, instead of paying taxes on his personal capital gains, he's placing them in an LLC, which he controls, ostensibly to do philanthropic work.
He could avoid all criticism by simply paying his taxes.
It's not like I don't try to pay the least amount of taxes I can every year.
(Avoiding or resisting taxes is also not always a purely self-interested endeavor. It's a tried and true political protest technique.)
I've got my own idea how to fix the situation. Could almost be done as a start up - feel free to use the idea - we need some sort of (ideally official) global accounting body that estimates how much profit each large international company makes globally and what percentage of their sales are in each country. So for Facebook that might be $4bn global profits, 42% US, 5% UK, SomeRandomTaxHaven 0.01% etc. Then all the politicians in any country have to do is say companies have to pay tax on the larger of either their local tax return or the international pro rata figure, so in this example the UK would tell Facebook to pay corporate tax on 4000*.05 = $200m or they'll ban them.
I'm not accusing him of illegal tax evasion. I'm accusing him, and Facebook, of legally not paying taxes. That it's legal, though, is of little consolation. Remember that it was legal to send Rosa Parks to the back of the bus, at the time. The criticism is directly that its legality is a poor veil and its morality open to debate.