I think the sites are safe but certainly brand loyalty is being created in subtler and subtler ways. The idea of targeting the young is nothing new but parents need to be having discussions with their children about the advertising so that children are aware what is an ad and what isn't. I worry that parents may be as naive as their children if they haven't experienced Second Life themselves.
Children do need to be taught online safety. However, unless a student arranges to meet someone that they meet online or divulges personal information it's not possible for someone to reach through the screen and grab you.
I agree with you on this point Nick. The dangers are different, but nevertheless as ominous. As a society, we actually support the advertising evils by buying into them (Xmas must-haves, Valentines dramas, etc). We end up being little thoughtless automatons on many different levels (self included, as I sit sipping my SlimFast), which is one of the ways we're directed around to our values systems.
Giving in to the ads problem is precisely giving into a predator, albeit another sort.
Both topics need to be addressed face-on; developing critical thinking is simply learning how to be aware of your surroundings--ALL surroundings. Evaluation is essential for all citizens.