Users of the social networking site Twitter should remember this axiom: Think twice, tweet once.

Unfortunately, many Twitterers believe, erroneously, that their posts are only visible to their followers. To the contrary: “The default setting on Twitter is to allow the entire world to view posts. But hardly anyone knows that,” says Carsten Ulbricht, a lawyer and Web 2.0 expert from Germany.

One Chicago resident has even been hauled into court after she insulted her building’s management company in a tweet.

Those wanting to limit access to their tweets to a more limited circle of users must change their profile settings. “I can set it to: Make my tweets visible only to my followers,” Ulbricht explains. That also makes them invisible to search engines, says Juergen Kuri from Germany’s c’t computer magazine.

But even if the entries are limited only to followers, users are still advised to think twice about how they formulate things. In countries like Germany, for example, those posting tweets about third parties must distinguish between opinions and statements of fact.

“I can write ‘The burgers at fast food joint X don’t taste good to me’ because that’s my opinion,” Ulbricht explains. More risky is publishing unsubstantiated information on the internet, such as the claim that burgers from that restaurant contain insects.

“If you want to claim a fact, particular if it is detrimental to a company’s business, then you have to be able to prove it,” the lawyer says.