I understand the need to protect your child. And I am not trying to discourage from getting a software that will allow you to track his.her online behavior. But you also need to understand that kids can create accounts on other sites without your knowledge.
If you have yahoo, they can easily go to gmail or some other website and create another email address that you know nothing about. We can monitor them and talk to them about the dangers of the internet and the predators but they know so much more about computers than we do.
I hope you find something that works. I am sure there is something out there but I would ask if it includes looking for all screen names or just the ones you know of.