The screenshot was taken here, and I believe it’s made using this (and this!). I don’t think it’s exactly what you’re looking for, but short of heading to the marketplace, it might be your best bet.
I don’t have any empirical data to back this up, but I would assume the average Facebook post is shorter than the average Discourse post - so while seeing ‘Read more’ over there is relatively rare, it’s likely to be more of an issue on your site. That might be a necessary evil, though.
A custom plugin may mitigate that as the layout would be optimised for your usecase, so more text could comfortably fit in the same amount of screen real estate, but it might cost a lot.