Facebook co-founder Mark Zuckerberg's sister evidently tripped on the social network's privacy settings, landing in the midst of a debate on Wednesday about "online etiquette."

Randi Zuckerberg, who launched a Silicon Valley themed online reality show after quitting her job handling Facebook public relations, kicked off the controversy after a family photo intended for friends went public.

The picture, copies of which were at Buzzfeed.com and elsewhere on the Internet, showed Mark Zuckerberg in a kitchen with family members dramatizing reactions to messages sent with a freshly launched "Poke" feature at Facebook.

Poke lets people send messages that self-destruct in what is seen by many as a spin on popular smartphone application Snapchat.

Some have joked that Poke is a boon for "sexting" risque pictures because senders can have them quickly erased.

Randi Zuckerberg posted a copy of the family photo to Facebook for the eyes of close friends only, but evidently it was also shared with friends of those tagged in the picture due to privacy settings at the social network.

That meant the fun photo popped up in the news feed of someone outside Randi Zuckerberg's circle, who then shared it on popular messaging service Twitter.

From there, the photo went viral -- much to Randi Zuckerberg's chagrin.

"Digital etiquette: always ask permission before posting a friend's photo publicly," Mark Zuckerberg's elder sister said in a Christmas tweet. "It's not just about privacy settings, it's about human decency."

The comment sparked heated debate at Twitter and other online forums, where a vocal contingent saw poetic justice in Zuckerbergs being exposed by the way the social network handles the privacy of users.

"How terrible that someone might take something that belongs to you and use it in ways that you had not anticipated, and for which you had not given explicit permission," Dan Lyons said facetiously in a post at ReadWrite.com.

"What kind of world are we living in when just because you post something on a website, someone else can take just take that stuff and do things with it?" he asked rhetorically before finishing with "Oh, wait..."

In a Twitter message on Wednesday, Randi Zuckerberg said the topic of online etiquette elicits "passion, debate, anger & Twitter crazies" to the extent that it might be the subject of her next show.