Well, I paid a mere $7.00 to Facebook to promote my book, to see what would happen.
I had a pretty good idea that Facebook would not do that well, especially after looking at my options:
I was not able to constrain my age range below 13 as that's the youngest that Facebook will let you manage a page or open an account. So right there, they are wrong for my demographic. I could only do Canada, as broadening the geographic range would make it more expensive than the experimental nature warranted, especially given the incorrect demographic, but the paperback is only printed in the US and UK so there's no free shipping, regardless how many your buy, so only the eBook is even a rough candidate.
In the end, they reached 329 people (actually pretty good reach for $7.00), of which only 8 clicked on the link (about $1.00/click) and only one shared the post. Clicking on the link meant they clicked on the Facebook post that had further links out.
Unfortunately, I know who some of the clickers are, so they would have done so from other sources, too, so a couple of those 8 clicks can be scratched, as they are meaningless.
All in all, if I ever want to sell something compelling to the general public, 13 and older, in Canada, Facebook is a great option! For my children's Christmas story, not so much...
So, my next step is to start digging into Amazon Advertising to see what the options look like.