Until now users seemed to think they had free rein to tweet whatever they wished.
But in a landmark case, Declan Ganley has forced an apology from an Irish blogger, Kevin Barrington, who posted defamatory tweets about the businessman last month.
Barrington was made to apologise and make a "substantial" donation to charity. Ganley's action is an important one, all the more so because it came weeks after TD Shane McEntee was abused on social networking sites.
As we know Mr McEntee subsequently took his own life. Declan Ganley's action was a civil one. But how long will it be before we see a criminal investigation into postings on Twitter?
It was recently claimed that death threats were made against Government ministers on social networking sites.
A death threat made in such a manner is no different from one made in person or by a phone call, and laws exist to prosecute such people.
We may see such a prosecution before long. Either way the internet's Wild West looks like it's about to be tamed.