If your kid spends a lot of her/his time just behind computer, using Internet its enough to be worried. But at least, you may think, they're not hanging out with some hooligans or boozing with friends under the city bridge. In fact there are many bad things can happen to kid, without someone visiting them or without them leaving the house.
My question considers adolescents, so let's assume just seeing a bit of nudity/violence or getting scared would not be devastating to them. I rather mean some longer processes, like a continuous on-line acquaintance with improper people, or something like that. Can you give some other examples of such dangers?
I'm sorry that my question is not fully in Q&A style, but I think I can get many concrete, interesting and useful answers.