I have a few thoughts about this. I have lots of experience with Facebook ads by the way. First of all, you have to remember that people who aren't subscribed to Medium only have access to three free articles per month, so if they have already read their free articles and then click on your post, Medium won't actually let them read it. In this case, you'd be better off publishing the same story on your own blog and then send the ad there. You can always put your Medium affiliate link there so that people can follow you on Medium.

Secondly, when running Facebook ads, you need to have a crystal clear goal in mind of what you want people viewing the ad to do. It looks like your ad is an engagement ad, which Facebook optimises for internal engagement, ie, likes, shares, and comments, more than to get you people who like clicking on blog posts to read. However, usually, when wanting traffic to your blog, you would want to create a traffic ad, or if you are looking for people to become leads, ie, opt in to your free subscription, or sales, you would want to optimise it as a conversion ad. And finally, I do think that Facebook's click-through rate is not reliable. It may be that people are clicking, the post isn't loading fast, and then they click off again. Who knows? Whatever is happening when they click or don't click, the numbers cannot be relied on. I think that the real question is whether or not the extra eyeballs on it did anything for you. Did it increase your revenue, create more comments on your post? If it helped create greater reach then I would say it was a $5 well spent.