Facebook founder Mark Zuckerberg has joined the growing ranks of billionaires pledging to give away all or most of their fortunes. Zuckerberg and his wife Priscilla Chan last week celebrated the birth of their daughter Maxby announcing that they would eventually donate 99% of their Facebook shares starting with up to $1 billion a year in the coming three years. The shares are worth about $45 bil lion today. In an open letter to their daughter, the couple said they wanted to"advance human potential and promote equality for all children The donation comes at a time when the world's wealthiest business leaders seem be challenging each other to give away their fortunes before they die. Zuckerberg also joined Bill Gates and other tech titans last month to promote clean energy and climate research. In 2010, Gates and Warren Buffett launched the Giving Pledge to encourage billionaires to donate the bulk of their wealth to charity. Gates has given away $31.5 billion and Buffet $22 billion so far. More than 130 billion- aires have joined them, including Saudi Prince Alwaleed bin Talal, who said the pledge inspired him to eventually give away his entire fortune of more than $30 billion