Maxima Chan Zuckerberg is held by her parents, Mark Zuckerberg and Priscilla Chan Zuckerberg shortly after her birth in 2015. File photo: Reuters

San Francisco - Facebook founder Mark Zuckerberg and wife Priscilla Chan, 32, are expecting again.

In a Facebook post, Zuckerberg, 32, announced the two are expecting another baby girl after the birth of daughter Maxima (Max) in 2015.

"I cannot think of a greater gift than having a sister and I'm so happy Max and our new child will have each other," he wrote Thursday. "I grew up with three sisters and they taught me to learn from smart, strong women."

After the birth of their first child the Silicon Valley power couple vowed to donate 99 per cent of their Facebook shares, worth billions, to charity in their lifetimes.