- Don't tell people exactly what you're doing, where you are, or where you're going, whether it's on Twitter, Facebook, or via iPhone. First of all, nobody needs to know where you are in broadcast format anyway -- it's just noise. Second, and more importantly, one must assume that the people interested in knowing where you are have some kind of incentive to care. Your friends may have perfectly benign incentives to care (maybe they want to meet up with you at the restaurant), but when you broadcast that information to the rest of the world, the only other people who might have an incentive to care about your location are those with nefarious purposes in mind: Stalkers, burglars, and the apparatchiks of police states. Don't give them more information than they should have. Tell people where you have been -- but only occasionally, and never in a way that signals your patterns and habits -- but don't tell people where you are.
- Assume that everything you've ever posted online is flashing on a digital billboard along a busy city street. Because, metaphorically, it is. And because, practically, it could be.
- Expect that there will be unintended consequences. Posting your grandchild's photo in place of your own on Facebook could expose the child to unwanted publicity if your name hits the news. And that comment that you made on a message board about a competitor five years ago could haunt your job prospects after a layoff tomorrow.
- Trust nobody's promises of privacy. There are too many ways to corrupt the system or break in through back doors to keep those promises alive over time.
- Don't build your house on rented land. Keep copies of anything you create, because the services you use could shut down anytime.