Do they really think that if some ISIS guy flies in, he's going to fill out the customs form honestly and say "Yep, my Twitter handle is @jihad4lyfe!"
No, they probably think that he will refuse to give them a handle and then they will use this as grounds to detain him for interrogation. The problem is that some of us don't use Facebook, Twitter etc. and so I don't have an account to give them - other than a dummy Facebook account which is entirely devoid of any personal information and that I only created because our local airport used to insist on Facebook to access the free WiFi. However I expect this will look like I created a dummy account to hide my real account from them.
I'm beginning to wonder whether the US government's long term plan is to make travelling to the US such a horrible experience for us foreigners that identifying the terrorists will be easy because they will be the only ones insane enough to try.