Consequences of having your personal details on the internet and how to avoid that

I've notice that some persons are not careful enough about the personal details that they release to the internet, and this is something that worries me. Most PC users don't really have the knowledge that we (computing students) have. Once in a while some websites pop up on my computer asking me for details and to me these might look dodgy, but to someone who doesn't have the same amount of experience, they might think its a safe website... And even if it is safe, there are some personal details that you just don't want to have in public sites like Facebook for example, Facebook is a really good place to find out people's address or date of birth, full name, phone number and things like that. it might sound harmless but you never know who's on the other side of the computer. Don't think you would like to have people ordering pizza to your place every 15 minutes, and you would need to pay for that... and this is just the minimum of what can happen.

Here is a example of what can happened if you're not careful with the personal details that you let others to access.