One of the very cool things about students using blogs is that it gives them a voice that may have otherwise not have been heard.
Some posts about Thing 3 at the RESA 23Things blog indicate a concern about predators and the like.
One should just do a cursory search for internet predator statistics and you will find many story's that really debunk the myths that 1 in 5 kids are approached by bad people' or whatever it is this week.
I, too, once thought this, until I started talking to my kids about their posts on Myspace/Facebook/Bebo/XANGA/Etc. By in large, they are very careful about their personal data.
This all comes down to ensuring that you are adequately supervising your kids activities online, just as you do when they are playing in the yard... and yes, it can be done.