Mark Zuckerberg, founder and chief executive of Facebook.

New York - Like so many of the network's billion-plus users, Randi Zuckerberg, sister of Mark and Facebook's former marketing director, probably thought nothing of posting a family snap on her brother's website.

It showed an assortment of Zuckerbergs in a kitchen with exaggerated expressions as they toyed with what was reported to be Facebook's new Poke smartphone app. The billionaire sibling was in the background, looking somewhat embarrassed.

But much to Ms Zuckerberg's chagrin, what was meant to be a private picture, intended for her friends' eyes only, ended up on Twitter when it was shared by Callie Schweitzer, a New York-based marketing executive at Vox Media, who saw it in her Facebook news feed. Although she is not one of Ms Zuckerberg's friends, Ms Schweitzer is an online buddy of one of the other family members in the photo.

When said member was “tagged” - Facebook parlance for identifying people in images - the picture popped up on Ms Schweitzer's feed, leading her to think it was public.

“Not sure where you got this photo,” Ms Zuckerberg initially said in a tweet to Ms Schweitzer, who as of yesterday had more than 40,000 followers on the micro-blogging website. “I posted it only to friends on FB. You reposting it on Twitter is way uncool.”

The Twitter back and forth has since been taken down, but according to excerpts doing the rounds online, Ms Schweitzer responded with an apology, saying she was “genuinely sorry but it came up in my feed and seemed public”. The digital penny seems to have dropped when the link became apparent.

“I think you saw it [because] you're friends [with] my sister (tagged),” Mr Zuckerberg is reported to have said. “Thx for apology. I'm just sensitive to private photos becoming 'news'.”

The curious case of the Zuckerberg snap has elicited comments from critics of Facebook's privacy settings. - The Independent