Spike has known Pinkie Pie for so long that her casual violations of normality almost don't even faze him any more. The operative word there, of course, is "almost". So he only recoils in shock when Pinkie stretches down to greet him, rather than screaming in terror on top of it.

"H-Hey, Pinkie," he manages, once his heart rate has calmed down. "You found this place too, huh?"