Before I wade through all the pat answers and get into the behavioural aspects of this "monster" if you are a parent here are some quick suggestions.

Enter into a dialogue with your kids. They need to know the pitfalls and hazards. Don't have a cow...educate.

Learn about the tools they are using and the places they like to hang out in. You have to technically know what is going on. You might not be as smart as them but you have to be that smart.

Keep the computer in a central room where what is going on can be seen.

Have a clear agreement in place. On what can be done and what cannot be done. They have to know and agree to the rules. They have to know the score it they are going to play the game.

You can filter if you want but filtering is not enough. You have to be involved !