A best selling traditional trad published offer has been twitting about her goodreads giveaways. She, well the publisher, is giving away 100 print books and has over 6000 people requesting it. You don't have to do 100 books, I saw one on the list with only ten. There surely are people who enter everything, people who like lotteries. I'm not sure where I'm going with this, I just thought it was interesting that the trades think it is a good marketing technique.
Edit: maybe it's less about the reviews and more of a word of mouth thing. People enter the lottery, put the book on their list, it shows up in the goodreads lists they belong to. Just speculating.