Zuckerberg donating 99% of his wealth to charity?

In a very long letter to his new born daughter yesterday, Facebook founder Mark Zuckerberg and his wife Priscilla Chan pledged to donate 99% of his Facebook share currently worth about $45billion to the work of their charitable foundation in her honour. Mark wrote;

As you begin the next generation of the Chan Zuckerberg family, we also begin the Chan Zuckerberg Initiative to join people across the world to advance human potential and promote equality for all children in the next generation. Our initial areas of focus will be personalized learning, curing disease, connecting people and building strong communities. We will give 99% of our Facebook shares -- currently about $45 billion -- during our lives to advance this mission. We know this is a small contribution compared to all the resources and talents of those already working on these issues. But we want to do what we can, working alongside many others.