Download and install K9 Web Protection from Bluecoat. It's a free program that gives all sorts of expanded parental controls and web filtering options. One thing it can do is block all social media sites (Facebook, Twitter, etc) as a category or you can blacklist specific sites. There are also timers for internet access where you can specify that between certain hours, allow access for a specified length of time, block internet access completely (such as when you're not home), etc.
It also logs and monitors all traffic and can send you email alerts when certain categories are attempted and such. I've used it for years. It can be as restrictive or permissive as you like (there's even the option to simply have it log all traffic but not filter or restrict it in any way).
Of course, all of this comes with the idea that as your child gets older, you need to talk about these things and give them ways to prove themselves responsible and trustworthy so they gain more and more freedom (within age-appropriate limits of course). If all you do is shut it down and not have good discussions with them, you're likely to just create resentment and nudge them toward finding ways to get around your restrictions (hopping on an unsecured WiFi outlet nearby, using a mobile phone as a modem, using their friend's mobile phone or internet connection, etc.)