I know it is hard to talk about "strangers" with your kids without instilling fear in them, but all kids need to be aware of people around them and know what to do if a stranger approaches them in a public place, like on the Internet.
There have been several news reports lately about attempted abductions of young girls, and in at least in one case the girl was wary, resisted the approach, ran, and was safe. Young people need to know that they should be aware of strangers, report them to adults if they see someone at school they do not recognize, and that if a stranger tries to talk with them, or touch them, they need to run to the nearest business, building, or even home (although at a home, they should know not to enter it) - asking the first adult they see to call 911!
Kids approached by strangers need to be "loud, large and in charge," to keep themselves safe! It is sad that our world requires they know these skills, but they do! Talk to your kids! The McGruff web site has a nice game about Stranger Danger to help with younger kids.