Often used by authors in romance and other genres where readers are predominantly on Facebook. Not likely worthwhile unless you have a well established Facebook presence already.

• How to run a Facebook ad campaign: https://mailchi.mp/hotsheetpub/podcast-book-deals#mctoc4
• How authors can leverage Facebook ads to sell more books: https://www.janefriedman.com/how-authors-can-leverage-facebook-ads-to-sell-more-books/
• Getting onto the USA Today bestseller list using Facebook ads (in part): https://mailchi.mp/hotsheetpub/usa-today-bestseller#mctoc3