Difficult one, there is only so much control you could ever have with that kind of site.
There are all kinds of ways of hiding information (eg stopping certain people from seeing certain information), you can't control who she chats to via message or messenger etc... or elsewhere on the web. I guess there has to be an element of trust involved (making sure they are properly educated as to potential risks,internet safety etc... and knowing they can come to you with anything they are concerned about).
With the availability of the internet these days, it's likely they could easily access it without your knowledge in any event should you try and stop them. Could be easier to keep an eye as much as you can with a willing and informed child who will let you.