After nearly two weeks, I've reviewed the stats from each ad campaign (Amazon and Facebook) and came to the following conclusions...
Facebook averaged out to about $0.50 per post engagement (a like, click, or share). Mostly likes, very few link clicks for purchasing the book. For link clicks, it was enormously inefficient, averaging over $15 per link click.
Amazon was very cost-effective, as no money is spent unless someone clicks your ad. For $5, over 30,000 people saw the ad, and over a dozen link clicks.
Goodreads also supports an author dashboard, and you can cross-promote with your facebook page and website, and sync your blog if you have one. Goodreads also allows you to do ad campaigns similar to Amazon.com.
It's still unclear after all of this if the ad campaigns are at all helpful. After turning them all off for a few days, ebook sales and Kindle Unlimited page reads have been relatively steady per day. So far, I'm not convinced that they're worth it. I guess the take-home message is that ad campaigns through Facebook and Amazon haven't led to increased readership for me...at least not with my limited marketing budget.