Facebook's co-founder Mark Zuckerberg has urged his young daughter to play outside and enjoy the wonders of being a kid, in a heartfelt letter posted on his social media site.

The letter was published on Monday to announce the birth of his second child, August, with wife Priscilla Chan.

In the gushing missive, the parents speak about the magic of childhood and the importance of play.

The couple posted a similar letter to daughter Max in 2015.

The post, which appears alongside a family photo on Mr Zuckerberg's Facebook page, encourages the new arrival not to "grow up too fast".

"The world can be a serious place. That's why it's important to make time to go outside and play," said the letter, signed "Mom and Dad".

The call to discover a childhood outdoors is at odds with the aims of the billionaire's social media empire.

Childhood 'magical'

"I hope you run as many laps around our living room and yard as you want. And then I hope you take a lot of naps," the parents wrote.

"Childhood is magical. You only get to be a child once, so don't spend it worrying too much about the future," they said.

It marks a lighter tone from the open letter written to welcome Max, which reflected on the problems facing younger generations and how technological progress will drive change.

At that time, the couple said they would donate their fortune to the Chan Zuckerberg Initiative to make the world a better place for Max to grow up in.

The organisation has pledged billions of dollars to improve life for their children's generation with goals such as eliminating disease.

But the parents wanted to shield their latest addition from these kinds of concerns, telling their daughter not to worry about the future, adding: "You've got us for that."

"You will be busy when you're older, so I hope you take time to smell all the flowers and put all the leaves you want in your bucket now," they said.