I recently self-published my book French at 60, which was an education in both the effort of writing it and the complexity of then publishing. The next stage was to sell it, which I now realise is going to perhaps be the hardest part of the process.
As a first step I created a business page on my Facebook account and decided to take a ten-day promotion, which cost me £20, equivalent to selling about ten books based on my royalty payments. I wasn't overly hopeful but it seemed quite an easy thing to set up. I chose an audience based on the subject, language learning, adding France as another key word and targeting an older demographic, which again fitted with the intended audience. The promise was audience reach and 'Likes', which was no doubt achieved judging by the summary of the promotion. But viewing and liking is easy, whereas meaningful engagement is not so easily achieved. From the summary below you will see that engagement was minimal. I shouldn't be surprised since Facebook is, at the end of the day, a network of fleeting engagements without real depth. It's all about 'Likes'.
And as for book sales, well you won't be surprised to learn that it appears to have driven a nice round zero!