It used to be a far ruder "F word" that could get you into trouble: now it can be two of them together - Facebook and friend.
More people are finding out to their cost that someone they had marked down as a social networking pal turned out to be not so friendly after all.
One man, a chef in north Wales, has been fined for posting anti-English comments on his Facebook site, one newspaper has reported.
It came after one of his 240-plus Facebook friends printed out his bilingual English and Welsh post and showed it the man's ex-boss.
It is a common mistake, according to social networking expert Haydn Blackey, of the University of Glamorgan, to think that your online "friends" will keep their loyalty no matter what.
Far from being a private, even intimate means of communication, sites like Facebook are as public as the restaurant in which the chef's food was served, says Mr Blackey.
"The privacy settings on Facebook do depend on your ability to decide on who your friends are and what kind of relationship you have with them.
"You can make only certain people see only certain parts of your site, but for most people, anybody who is a 'friend' can access what you write."
He pointed out that people who are friends in the social networking sense are those you might have only an acquaintance with, and different from friends in the face-to-face social sense.
"In an online context, it simply means someone you may have a vague connection with or knowledge of - it might even be a contact of a contact," said Mr Blackey.
"While you can rely on your friends to deal with any confidences you give out and not to use your work against you, in the social networking context, it not quite like that.
"It's quite easy to be sharing stuff with someone who is a friend in Facebook terms but who does not have that kind of loyalty to you.
They may think 'I'm a closer friend to that other person than to you'.
"It does mean that the language you use needs to be much more focused because it's easy enough for someone to copy and paste or print out some of those things you are saying."
However, even those with a greater knowledge of the law can face criticism for their online posts.
Mike Stoddart, a former newspaper owner and now a Pembrokeshire councillor, was the subject of a complaint by members of Manorbier community council for comments on his blog, called Old Grumpy. However, he has now been told by public services ombudsman that the case has been dropped.
Mr Stoddart knew well that his comments would be read widely, and said he felt vindicated after the ombudsman's decision.
But what lulls others into believing that they can pour out their feelings or opinions to a network of cyber buddies without the rest of the world - and possibly the relevant authorities - finding out?
The key, says Dr Jane Prince, principal lecturer in psychology at the University of Glamorgan, and a researcher in online identity, can be where and how it is done.
She argues that, just as the infinity of the digital world is given a nice clean edge by your computer or mobile phone screen, so people's common sense can be curtailed when they are posting.
Dr Prince said: "In cyberspace, people seem to be quite sophisticated in their knowledge of privacy issues but they don't seem to let that knowledge affect their behaviour at all.
"People pay mental homage to issues of security and confidentiality - even children are very aware of issues around privacy - but then they behave in a way which seems to ignore that awareness."
The setting for postings, perhaps in the comfort of home or the ambience of a stylish public place, can play an important part, she said.
'So many loopholes'
"It is usually done in a safe-ish place - not the middle of the road, for example - and they can see privacy in that context."
Commercial litigation solicitor Tracey Singlehurst-Ward, of Cardiff-based law firm Hugh James, said people had the same responsibility to stay within the law when posting online as any paper-based publisher.
She said: "What I see increasingly is people bringing actions.
"What is important to remember is that you are not having a private conversation. It is public to the world, even when there are restrictions.
"Once it appears, there are so many loopholes to get on these sites, they are public and across the world."
It is not only lone Facebookers who can fall foul of the courts.
One case she has dealt with involved a Facebook political group site that was used by opponents to post what amounted to defamatory material.
"Group sites can be more dangerous because they are more open," she said.