-Colleges, grad. schools, and future employers CAN and DO look you up on the Internet
-Once it's out there, it's out there, and you can never truly delete something once it goes out into the public domain.
-You never know who is looking.
I think these are the kickers.
We're of, or at least cultivating, a generation that uses the internet as a playground. We're flippant, bolshy, opinionated, and we're carving it all into tablets of crystal and giving it to other people. Everyone's had stupid conversations in a pub, and everyone's said and done things that they might have regretted not hours later, not days later, not weeks later, but
years later. We might have been certain at the time, but we learn, and we grow, and change, and we're never exactly the same person we were a few years ago. Etching those conversations into the world's biggest public info-source is a little barmy.
That's not to say "don't use the internet." Do, it's great. It's more... I think it's important to think carefully about anything and everything you say. Fine to be flippant, but avoid knee-jerks, or confrontation. Bad to have a fight at school, where all your mates can see it. Worse to have that fight on the internet, where everyone might be able to see it for the rest of time. If it went viral - and look at websites like "failbook," these things
do go viral - you might only ever be remembered as "the girl who made the bad taste holocaust joke," or "the guy who sent an intimate message to the wrong person."