I think rockinhippy's said everything I was going to say!
Obviously, if you lack the time, skills or inclination to keep a regular eye on your child's internet usage, there's all manner of risks out there.
But it's because I teach secondary & know rather more than I'd choose to know about those risks, that I would greatly prefer to have my dc reasonably savvy about web safety from a relatively early age.
Like before they set their own account up when I'm not looking - my 5 year old son's quite capable of doing so, given 5 minutes unsupervised computer access.
OK, atm I pretty much do control his access - bar when he's at school, where the LA firewall automatically bars social networking sites (there's ways round that, though, & our ICT technician spends a couple of hours a week plugging those leaks).
But increasingly, as he gets older but WELL before he's 13, I'd expect not to be hovering over his shoulder every second he's online.
IME naivety lies more in assuming that if a parent hasn't agreed to their child creating accounts on networking sites, why then no such account exists.