Val's got it exactly right. It's not just about secrets, it's about control. You put something out there, even in a private group, and you lose all say over where it goes. Screenshots get taken, stuff gets saved, and then it just lives forever on some server you don't even know about. The internet has the world's worst memory for the things you want remembered and a perfect memory for the stuff you wish would disappear. Once it's digital, you have to assume it's permanent and public, because it probably is.