I have a feeling this is going to confuse so many people... for a while. And then they'll get used to it.
It's one of those design decisions that big companies don't usually want to make for fear of alienating so many technically uneducated users and I'm sure Facebook will hear many cries of "But how do I comment?!" and "How do I start a new paragraph?"
Or maybe I'm over-estimating the average Facebook user's ability to write more than a few works in txt tlk. LOL. j/k. :P
