Footprints in the sand are notoriously short-lived, in sharp contrast to the ones we leave on the net. It's a curious paradox that whilst many people are concerned about integrity and security on the net many of them show little practical interest in the problem. Facebook profiles are often wide open and many people tend to post comments and photos first and ask questions later (or never).
Here's an excellent presentation by Australian educator Jenny Luca called Nurturing your digital footprint. She presents examples of indiscrete use of social media and gives practical tips on how to take control of your digital profile. This is all essential knowledge in terms of digital literacy and relevant to all age groups.