Paying for advertisements increases the number of eyes that see your book, but there are other ways like hashtags and reaching out to people directly. I have always heard that the best marketing is always by word of mouth, and the goal is to increase the chances of that word getting started.
Goodreads, the Amazon-owned social media site for bibliophiles, introduced a major overhaul to its giveaway system in early January. Previously, giveaways were free to host, making them a popular and cost-effective marketing tool for indie authors and publishers. Readers eager to win free books would browse the giveaways page and enter any contests that caught their eye, resulting in lots of exposure for books that might not otherwise get seen. The only cost to the host of the giveaway was that of the book (or books) and shipping.