Stephanie Kahn wanted to bask in her engagement for a few hours before diving into the task of calling aunts, uncles and good friends with the big news. And even before she could call them, she had a surprise party to attend, one that her fiance had set up for their parents and her “closest group of girlfriends.”

That party was when Kahn lost control of her news. Some of the guests took photos and were “uploading them on Facebook before I could even post anything,” Kahn said from Smyrna, Ga., where she lives. “Of course the next morning I get a couple of calls, text messages from people I didn’t call. They found out on Facebook. I think some people were a little upset.”

In an age in which instant news and constant life streams from Facebook and Twitter change the way we communicate, the rules of etiquette surrounding these interactions are still evolving.

What happens when I expected a phone call about something and read about it in a status update instead? What’s the polite response to a distant friend posting bad news on Facebook? What to do with sensitive information?

Making matters trickier, good etiquette on Facebook might not apply on Twitter or in an e-mail. These days, milestones like marriage, pregnancy, breakups and divorce are being described over more forms of communications than ever.

Online social networks haven’t been around long enough to develop hard and fast etiquette rules, but general guidance is emerging. Just as most people learned that it’s annoying to yell on a cell phone in public or to hit “reply all” when responding to just one person in a mass e-mail, social media-savvy folks are finding it’s unwise to, say, post unflattering images of friends without their consent.

Etiquette adviser Anna Post, the great-great-granddaughter of manners icon Emily Post, recommends taking a step back before rushing to type, whether it’s good news about you or a response to someone else’s bad news.

A decade or two ago, communicating important news electronically rather than in a letter was frowned upon. Now an e-mail is considered acceptable for many situations, but even people comfortable with that might draw the line at social networks, which feel more like public or semipublic venues.

After all, the average person has 120 “friends” on Facebook, according to the company. In real life, the average North American has about three very close friends and 20 people they are pretty close to, said Barry Wellman, a sociologist at the University of Toronto. This means people may sometimes forget just who is reading their status updates, and can let their guard down.