I'm not completely sure why everyone is freaking out over this...it's essentially like having the Facebook app installed on your phone, only better...instead of apps jumping back and forth to Facebook, authentication, sharing, etc will be native to the OS...that's all...
Much like people who don't have Facebook installed, it will be like the many features you probably don't use...
For us developers, this is great news...twitter integration made life so much easier,...and iOS 6 cannot get here fast enough...I don't like all the hoops to jump through for FB integration and authentication,...plus overall it's just a poor user experience in my opinion...I'm sure Apple and FB have streamlined it all, to make it as seamless an experience as possible...
And if you don't like FB, then continue not liking it, because you'll never know it's there unless you actually TRY to use it...much like your calculator app...I don't see Sliding Ruler fans freaking out over the built in calculator app,...