This feels like maybe a [citation required] thing. Facebook doesn't have any particular length limit on posts, and anecdotally vast screeds are quite rare on my feed, and most posts are pretty terse regardless.
Is there data to suggest that people will just rant all the time if they're permitted to?
[edit] Anecdotally, my current top 20 posts in my FB feed: 17 under 140 chars, 3 greater than 140 chars.