This photo provided Aug. 28, 2017, by Facebook, shows Facebook CEO Mark Zuckerberg with his wife, Priscilla Chan, and their new baby daughter August (left) and her sister Maxima (right) in Palo Alto, California. On Aug. 28, Zuckerberg announced August's birth on his Facebook page. (CHARLES OMMANNEY / FACEBOOK VIA AP)

NEW YORK — Facebook CEO Mark Zuckerberg and his wife are welcoming a new daughter in a post — where else — on Facebook.

Zuckerberg and Priscilla Chan already have a toddler daughter, Max, born in 2015. They named their second child August.

The couple posted a photo of the family Monday, along with a letter they wrote to August hoping she will get to enjoy her childhood. They write that "childhood is magical" and tell her that she only gets to be a child once "so don't spend it worrying too much about the future."

Chan and Zuckerberg also wrote a letter to Max when she was born, talking about the world they hope to see her grow up in, with advances in science, medicine and access to education.