Specifically, Facebook built a messaging feature for New Year's Day. The feature lets users pre-write messages to friends and family and have them automatically distributed as soon as the clock ticks midnight and the New Year is official. It's a neat feature in concept, but in reality the messages would not have been private.

"By simple manipulation of the ID at the end of the URL of a sent message on the Facebook Stories site, you are able to view other peoples Happy New Year messages. At least I was when I edited the ID for myself," wrote Jack Jenkins, who discovered the flaw, in a blog post. "It is, you may say, a pretty harmless flaw, as they tend to be generic messages and you can't see who sent them (it shows your profile pic next to the message, as if you've sent it). However you can see the names of the recipients of the message."

Privacy Faux Pas

Jenkins pointed out that some messages do contain a photo. One such message he saw contained a photo of a father and their child, another a family photo, another was a personally written message. He offered screen shots as proof.

"I don't know who these people are, but you can see it puts my profile pic next to it, as if I have sent the message. It shouldn't be possible to do this, as these are not generic and are people's personal images," Jenkins said. "A very bad part of it all is I think that you can actually DELETE other people's messages, which I have tested for myself on a single message as I thought that it would say access denied."

Jenkins went on to describe how to delete the messages. Facebook got the issue fixed Monday morning, but it is still reportedly not working in some parts of the world.

Zuckerberg's Sister Angered

Facebook already was ending the year with a privacy-issue bang, though it was not all Facebook's fault. Last week, Facebook CEO Mark Zuckerberg's older sister, Randi, wasn't too happy when someone outside her inner circle saw -- and then tweeted to 40,000 people -- a family photo she shared on her brother's social media site.

The photo shows her sisters using Facebook's new Poke app, which is similar to the popular sexting app Snapchat, on their smartphones. In the photo, Mark Zuckerberg was looking on with a strange look on his face.

"Not sure where you got this photo," Randi tweeted at Callie Schweitzer, the director of marketing and special projects at Vox Media who tweeted the image. "I posted it to friends only on FB. You reposting it to Twitter is way uncool. I would hate for a private photo of mine to be public and would never want to do same to others."

As it turns out, when you tag people in a photo, by default those images can also be seen by any of that person's friends. You can change that setting by creating a "Custom" option under photo sharing. It's not confusing, but it's not second nature, either.