Social media is a great friend to self-published authors like myself. It gives you platforms to find friends, share your posts and encourage potential readers to learn more about you as a person and author.
That last part is significant. I think someone has already said about not spamming sites, especially promotion sites on FB for example. To get your posts noticed, you have to be seen as someone who will also encourage other authors in your genre, offer advice, and be friendly.
The most effective free way of promoting your books is to build an audience:
  1. Create your own personal group on FB or MeWe
  2. Encourage readers to join your group by posting a link to it at the back of your books.
  3. Make posts about things associated with your story - invite comments.
  4. Have competitions on your group and offer small but appropriate prizes.
  5. Post teasers from your latest WIP to your group - letting your readers know they are getting an exclusive look into what you are doing.
  6. Be yourself and share things about yourself, so your readers can see you are more than a machine that writes stories. I honestly think my puppies have more of a following on FB than I do lol.
  7. Friend and promote other authors in your genre - join author takeover events, offer copies of your books as competition prizes, and become known in your genre.

That list is only the tip of the iceberg of course, but hopefully, it will give you a start. Most of all, have fun with it. Marketing is a huge fact of life for all authors, especially those who self-publish. Of course, it helps if you’ve written a good book.
Have a lovely day :)