If you start scrolling down your news feed in the new, native Facebook app for iPhone and iPad, the post bar containing status, photo, and update buttons scrolls up with the rest of the content.
When you’re farther down your stream and start scrolling back up, the post bar will drop down at the top of your content, but only if you lift your thumb up after swiping down. If you tap, swipe down, stop, and lift, the bar won’t appear.
I wager the assumption is that when moving your content up, you still want to read. But if you start an actual scroll operation to go back up, maybe you want to get back to post something.