Unlike preschoolers and school-age kids, who look to their parents as primary sources of influence, tweens are in the process of shifting their attention from parents to peers. They aspire to grow up fast — to be teenagers — so they often start pushing boundaries. They need help understanding which behaviors are acceptable and which aren't, whether offline or online. They need boundaries set and expectations of increased personal responsibility made clear, since they want to spend a lot of time online, and they want to spend it alone.
First, continue to be a active parent in the media domain—it is just another part of the world in which you're raising your kids. And even though your tween probably knows more than you do about navigating technology, the part of his brain that controls impulses and understands future consequences isn't developed yet. That means that a big part of your job is to provide that structure and guidance for them.
You do that partly by figuring out the kinds of freedom for which your tween is ready. Sometimes they need the chance to take risks and fail. In other cases, they are not developmentally ready to make responsible decisions or the stakes are too high for the learning to be worth the risk. The challenge is to know which is which.