Zuckerburg's pledge aside, if he were to do nothing with his new LLC, he would face no tax penalties and would essentially be moving money from one place to the other with nobody benefiting in any tangible way.
The reason people are upset is because the last 30 years have amounted in an enormous transfer of wealth from the poor and middle class to the very rich. All that wealth - everybody's wealth - was created by the labor of people long dead, all the way down to the first humans to develop agriculture. A staple of any society has been the need to tax wealth creation so public goods can be provided, from national security to a functioning court system.
By dodging taxes (which is what happens when you make a "charity" that isn't structurally obliged to make donations) you are circumventing the social contract and depriving the public of a portion of the profits derived from being a part of that society.
Creating Facebook or any other large company with a big workforce is no excuse to not pay taxes. Those people use roads, water, courts, and require protection from the police and military. That causes additional strain on the system and has to be paid for somehow.
I don't care how he spends his money, but establishing an LLC as a charity is a poor form of philanthropy.