Here's the thing - On my recent release, THE MIRROR, I thought I'd try Amazon one more time because ... well Amazon. The results are disappointing. First off, when I actually did a search for Amazon giveaways (and how many people are aware Amazon does giveaways? I had to look HARD to find them), mine didn't show up. The giveaway must have shown up somewhere, because I did get entries. Yes, I shared the link on social media and promoted it. Most of the entrants appeared within the first couple of days. When I finally DID find how to search the giveaways page, dozens of unrelated items showed up in my search, so 20 pages of partially relevant items (none of which included the giveaway I sponsored).
Let's take this a step further. Of the winners, how many people actually read the book and leave a review? I ran an Amazon giveaway in 2015, and of the three winners, I received zero reviews. Okay, not everybody leaves a review, so I can't say I was disappointed, especially with a population of three, but these are people who buy and one might assume read books. This time, I did get one review that is "most likely" from one of the winners.
For the a giveaway at Goodreads, and generally I give away five books at Goodreads, I often receive reviews from the winners. Granted, Goodreads is a site designed for readers, but by that same logic (not everybody leaves a review), it might be reasonable to assume zero reviews again. However, for the GR Giveaways I've done, I've been lucky enough to receive, on average, reviews from three of those five people who've won.
Different people may have different experiences, but for me, based on the inability to FIND my giveaway on Amazon after the first week when doing a search, and based on the number of entrants, and based on the response of those entrants (i.e., reviews or sales to the people who didn't win), I'll stick with Goodreads.