*just my opinion*
A secure password will stop the majority of problems security wise, but as with most other things (car, house, business etc etc) if someones determined to get in they probably will. The real issue is that you are partly reliant on someone else for that guarantee, rather than your own senses and a bl***y big stick. I've read that part of *******s problem arose from the fact that the original site build never foresaw the level of traffic it got, so they just had to make it up as they went, rather than take it down and restructure it.
As for information control, I'd start by reading a sites T & C's. If they sell it on, claim any and all rights to it and offer it up to any passing hawker without your say so, it makes sense to be economical with it. You are in control of it - but only until it enters the system.
A 'dummy' Email is a very good idea for starters as has been said. Not getting caught up 'in the moment' and bleating about a particular issue is another - I suspect we've all done it but it makes sense to take a step back before hitting that button. Would you walk down the street shouting your personal opinion on a sensitive issue? I doubt it.
Bottom line, I think we'll find it increasingly difficult to operate without a 'web layer' to our daily comings and goings. It's not going to get any less complicated but it may get more intuitive, making it even easier for us to screw it up.