Well they are. They can suck and suck and your time disappears down a vortex never to return! I avoid Facebook as much as I can. I like Twitter. It basically displays content I want to read. LinkedIn I visit once or twice a week. After that I never really bother with any of them. And I think that is the key to managing your social networks. Visit the ones you want to visit but have an aim when you visit. If you simply just visit they will take up your time. But have an aim and you will be able to manage the time you use.
Of course it might not matter to you in which case simply forget ever reading this!