One bit of information leads to another bit until after a short time you use those pieces of the puzzle to get a larger picture. The child doesn't realize that what they are saying may put them in danger. If that teen thinks they are emailing with a teen of their own age, with similar problems and activities, they are likely to talk freely. This is something a predator is hoping for. Many young people have set up the ISP or online service accounts.
Most teens know more about computing than their parents.
These parents then do little supervision of what areas their child is exploring and have no idea how to control what the child is involved in. The computer should not be in the bedroom. It should be in a common room where you can watch what the child is doing. Just like the television, online time needs to be supervised. Sit with the child and watch what they are doing.