How does Reddit/Content based platform set their meta descriptions?

I'm trying to figure out a way for my website to systematically set our meta descriptions. The content similar to Reddit and Quora with community pages instead of subreddits.

I'm not sure what is the best plan of attack for setting the meta descriptions for these posts. Should I be using the first 160 characters, last 160 characters or somewhere in the middle from someone's post? I can't really seem to get good recommendations on how to systematically set our meta descriptions, because there isn't a preset amount of pages that exist.