Facebook does not allow you to build a custom audience of 1 person. I believe it has to be a minimum size of 100 or thereabout. Which means when Spinner says your target will get 180 impressions, they have to throw in 99 random non-targets into the targeted audience in order to get the placement. Thus they really aren't marking it up as much as you think.
They would batch them though, wait for 100 clients who want the same and put them in the batch. So it does provide a marginal service, viewed from that angle.
The less understandable part is that it's charging $50 for 180 impressions such as https://twitter.com/RichLeighPR/status/1037066048485371905 while Facebook sells those directly for a few dollars per 1000, i.e. 100x markup.