I can see why you might be concerned about solo YouTube wanderings. As a parent of a teenage boy (15) I'm not that worried about porn. It's more adults who model behavior I don't want him to learn, specifically trash talking women. And this kind of thing is very hard to tell when it's the audio in you-tube video about a computer game unless you pre-watch every last video they see. Filter's don't really work for this. This is why at this age, we mostly talk about what we expect and keep the computer in the living room so we can keep an eye on what he is doing. Most of what he does is computer game related.
The rule I had for facebook when my daughter was younger was that she could only "friend" people that she knew in real life. They didn't have to be close friends, for example classmates were fine. But she wasn't supposed to friend anyone she couldn't easily verify they were who they claimed to be. As she was a very compliant teen I never had any big problems with this. My DS isn't interested in social media sites like facebook, tumblr, twitter, or google+.
