I haven't seen any sites that REQUIRE you to have a FB or Twitter account.
That said, I do have a FB account that I use regularly (Twitter less so), and it is helpful for me to have my favorite sites aggregated. I like the single sign-on convenience when I can connect sites through FB. There is less information on FB about me than one could find by doing a simple Google search, so I don't consider having an active social networking account a loss of privacy in any way.
People who post when they're going on vacation or that they hate their jobs, deserve what they get. It isn't rocket science to remember to keep private things, private.
Meanwhile, potential employers can find me easily on FB and sites like LinkedIn, and if I want to continue to have a successful career that's critical. I'm going to be 40 in the spring, so I can't exactly afford to pass up opportunities to show others that I'm not 10 seconds from retirement.
I wish I were. But I'm not.