That's the current value of the funds Mr. Zuckerberg and Dr. Chan have announced they will dedicate to social purposes. In doing so they are sending a clear signal that they recognize the choices available to them (and, indeed, all of us) and have chosen to use them to pursue their interests. The Chan Zuckerberg Initiative, a Limited Liability Corporation, puts all of these choices in one vehicle.*
Despite the timing (GivingTuesday) and the headlines, what the couple announced does not constitute a gift or a charitable donation. It's inspiring, may produce positive social good, may usher in a new era or serve as a watershed moment for the new social economy, and is certainly an important and impressive act. But what they did was set up an LLC. They didn't transfer the possession of any assets, nor when they do shift assets to the LLC will they have relinquished possession or control.
*Possibly not crowdfunding, but who knows.