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.

Nobody's arguing people will rant all the time. A lower frequency of longer texts is expected, even if Facebook didn't show it.

Does this include adding 23 chars for a link, and possibly also for images (which I think used to count as extra but no longer do)?

We don't need twitter to be another facebook.

Not sure why you've been down-voted. Twitter is so different from Facebook in terms of what it's primary use is, that it doesn't make sense to compare them, and while I wouldn't quite word my response this way, I agree with the sentiment that Facebook usage data doesn't tell us enough to predict Twitter usage under higher character limits.