One of my former students told me: “Since June, I’ve been spending as much time marketing my ebooks as I’ve been writing. It’s working. My sales dipped slightly in August, but they’ve come right up again.”
He’s using social media for his marketing, mainly free promotions (other than the time he spends), as well as spending a small amount each week on Facebook advertising.
In social media (Twitter, Facebook, Pinterest etc) promoting your books works, as long as you work at it. Remember that social media marketing is social. You need to spend as much (and more) time on social activities on the networks as you spend promoting on those networks.
Vital: don’t wait until your book is published. Start promoting when you start writing your book — get subscribers to your mailing lists.