Mark Zuckerberg's sister and Facebook co-founder, Randi, expressed her disappointment when a personal family photo she posted exclusively for her friends was shared to the public in the other social networking site "Twitter."

An upset Randi Zuckerberg stated on her Twitter account @randizuckerberg: "Digital etiquette: Always ask permission before posting a friend's photo publicly. It's not about privacy settings, it's about human decency."

Callie Schweitzer is the identified Twitter user who shared the picture on her account @cschweitz. The photograph shows members of the Zuckerberg family reacting to "Poke" on their smart phones. "Poke" is the latest mobile application that Facebook developed similar to "Snapchat."

To further express her dismay about the privacy issue, Randi Zuckerberg sent a Twitter message directly to Callie Schweitzer. "Not sure where you got this photo. I posted it to friends only on FB. You reposting it to Twitter is way uncool," Zuckerberg wrote to @cshweitz.

Callie Schweitzer apologized and explained that she follows Randi Zuckerberg on Facebook and stumble upon the picture on her News Feed. Randi Zuckerberg afterwards stated that she already accepted the apology.

According to Randi Zuckerberg, there is also this possibility that Callie Schweitzer had access to the personal photo since Schweitzer is friends with her sister who was tagged in the personal family photo. Now, the controversial picture as well as Twitter messages has been deleted.

Randi Zuckerberg, who began a Silicon Valley-themed online reality show after leaving her job in managing "Facebook" public relations, sparked debate at online forums regarding the privacy settings concern.

"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, Editor-in-Chief of tech blog ReadWrite, wrote.

"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?" Lyons added rhetorically.

What happened to Randi Zuckerberg further placed "Facebook" in a predicament. Several users have been complaining about the confusing privacy settings of the famous social networking site.

"This is absolutely about privacy settings. That person shouldn't have seen it. Your brother's site is doing this to us all!" an unnamed user reacted to Randi Zuckerberg's Twitter message.

Meanwhile, Randi Zuckerberg posted a Twitter message confirming that she will still tackle the privacy settings issue on her next show. The subject of web site etiquette will surely draw out debate from millions of angered Facebook users and Twitter enthusiasts.