There is this phenomena called sleep paralysis,which can be recurring or isolated. It happened to me once and scared the piss out of me. Basically, this occurs when your body is awake but your mind is still dreaming. Your body is unable to move, you feel pressure on your chest, you may hear footsteps or voices or see apparitions. This usually happens right as you're falling asleep or right when you wake up. When it happened to me, I heard a woman's voice calling, "Ami, wake up!" over and over. When I opened my eyes I was unable to move and I felt like I was suffocating. Terrifying.
Likelihood: Meh. If Ke$ha has recurring instances of sleep paralysis, it's possible that she's mistaking it for ghost sex. But having experienced it myself, and reading up on other accounts, it's not a pleasurable experience. And Ke$ha seems to have enjoyed whatever she did with her ghost dudes.