What happens when someone shares a dark post, or for that matter, any newsfeed post you paid for? Does it get seen by all of their friends, a % of their friends, or what?
Just like with any post from anywhere, it will get shown to friends with social context.
Because of the competitive nature of the news feed and lack of space, these will get a low % delivery.
E.g. if you get 100 likes you might get 10-1900 social impressions…