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? Making matters trickier, good etiquette on Facebook might not apply on Twitter or in an email.

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.

“The word Facebook uses, ‘friend,’ of course isn’t true,” Wellman said. “Many people Facebook calls friends are not friends but maybe acquaintances or former friends.”

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.

Indeed, tweets and status updates posted in the heat of a moment can quickly backfire. In July, a New York City government aide resigned after posting inflammatory Facebook comments about the arrest of Harvard professor Henry Louis Gates Jr. The aide, Lee Landor, had called Gates a racist and referred to President Barack Obama as “O-dumb-a.”

The lesson?

Know your audience, especially if they will complain to your boss.