Two friends innocently posted pictures of themselves posing suggestively in mock school uniforms on a night out on Facebook. Just like millions of other users who do the same thing every day on the social networking site, the pair were sharing their experiences with friends.

But, there was a problem. Both women, whose pictures were easily accessible to anyone, were teachers in an Australian school. Outraged, local newspapers published the photos. Authorities got involved. And the incident gained international notoriety.

Stories about how sharing information online can quickly get out of hand are rife. Every Facebook user probably sees status updates they would rather a person hadn’t posted, or a picture which makes someone look bad.

So just how public are Twitter and Facebook? And, what precautions should you take when posting to the website?

Beware of Your "Digital Footprint"

Have you ever posted a disparaging comment to Facebook about a boss, work colleague or neighbour? Chances are, if you have, it could easily be found, and it could come back to haunt you.

Imagine applying for a job and a prospective employer happens to find your posting about your ex-boss. What does that say about you?

Webwise promotes internet safety and as part of that we believe all internet users should think before they post.

Irrespective of your privacy settings, anything you say or post online can become publicly viewable. Once you publish something, it can easily be copied, shared and altered.

Private or Public?

A lot of people don’t know that upon singing-up to Facebook as an over 18-year-old, your privacy settings are automatically set to public.

If you want to change this, you must customise your privacy options and opt-out or disable settings if you wish to retain some control of your identity and online reputation.

But even if you do this, there’s still a chance that what you post online could become public. Say for example a friend retweets a picture you tweeted and his/her profile is public? The same thing can happen on Facebook.

And that’s not all the sharing that takes place on social networking websites.

Take Facebook for example. Its revenue – it’s a business after all – is not generated from users.

Instead, Facebook pools the information from profiles, yours included, to discover what types of things people like. It then offers to pitch a business’s product to a group based on the information it gathers, and commands vast fees for doing it.

Billy Hawkes, the Irish Data Protection Commissioner, recently told a seminar at Dublin Institute of Technology, that the implicit agreement between a user and Facebook, given that the service is free, is that you give Facebook personal information about you so it can garner money for pitching ads at you.

Young People and Social Networking

Facebook insists that user under 13 are banned. In reality, however, the ban is pointless.

Significant numbers of Irish children aged between 9 and 13 have profiles on social networking websites like Facebook despite the rules, according to the Social Networking Among Irish 9-16 year olds report, conducted by Dublin Institute of Technology researchers.

The report revealed that half of eleven and twelve year old children, and one in five children between the ages of nine and ten, have social networking profiles on either Facebook or Bebo.

Worryingly, the report also revealed that 18 per cent of Irish children aged between nine and ten - the highest of any age grouping - have left privacy settings untouched, meaning that their profiles are public for anyone to see. This drops to ten per cent for boys and girls between eleven and twelve.

And the same risks apply to young people online in terms of privacy.

In short, nothing you post online is private, everything is public, and young people as well as mature users should always remember this.