Offering free e-Books can be a very powerful promotion tool, but only if you use at least one social network, where you inform the potential readers that your book is available for free within a certain period. Cheryl Kaye Tardif succeeded in this way, as described in her book How I Made Over $42,000 in 1 Month Selling My Kindle eBooks and she herself admits that she can ascribe 90% of her success to Twitter. So, without Twitter she would not have succeeded as well as she did. And as she had spent quite some time on promotion before, she had a considerable number of friends and followers.
A few days before she began her promotion, she started creating a buzz and during her promotion she sent a tweet nearly every hour. She worked very hard after the promotion as well, informing her friends and followers about her sales.
I have noticed that the KDP select program on Amazon no longer works as well as it did in the beginning. The number of free books available is huge so it is very difficult to succeed without at least one social network.
Another condition is that you have a large number of friends and followers who will send your recommendations to others. It is no use having 10,000 followers on Twitter who never read your tweets or do not care what is happening to you. One of the main conditions on Twitter is that you have a number of friends and acquaintances you communicate with (and not just send promotional tweets about your book), you know something about them and they know something about you, you retweet their posts and they retweet yours. In this way you create a mutual relationship where you help one another.