Yep. Facebook was my bread and butter. Because my book (http://www.amazon.com/Trail-Magic-Art-Soft-Pedaling-ebook/dp/B00NJQZ6GK) is niche, I made my own Facebook group and invited five people. Over the course of three years it's up to 3000 people... in fact most folks don't even realize I run the admin there.
Since I have 3k people, I'm able to advertise my book in the banner section. Also, I drew all my illustrations in my book so I was able to spread those around on Insta, Pinterest and Twitter (I have a decent base on Twitter, but not as much). In the book I tell people I'm rather chatty online and would be happy to talk to them about the stuff in the book. They do, I respond.
Also, when people say "I read your book, I loved it!" I ask them to write a review at Amazon, which is nice because I'm up to 51 reviews.
I also know "my people" and what forums they hang out in, so I will - when it's appropriate - plug my book (I just don't come in spamming left and right). All this has garnered fairly good sales (I'm not getting rich, but I'll be able to buy gifts for xmas).
I engage my audience on Facebook (once again, niche... it's endurance bikepacking and racing on a mountain bike... I wrote a book on a huge race from Canada to Mexico on the backroads and trails of the continental divide finishing 4th place on a singlespeed... the kicker is, when I started training I had barely been on a bike and was fairly overweight and sedentary).
Lastly, I made a landing page for my book that has helped (www.softpedaling.com) along with a nice youtube opening (https://www.youtube.com/user/sthig).
I market for at least an hour a day (on average) and honestly it's more work to market than it is to write, but I don't mind it because I believe in my book.
I also contacted every media outlet I could find (bikepacking magazine, the american cycling association, bike magazine, bike blogs) and said "I wrote a book." Since most people in this genre don't write books, it's a little more open market than say "I wrote a children's book" which the market is saturated with that.
^That's ^why ^I'm ^here, ^I ^don't ^judge ^you. ^PM ^/u/xl0 ^if ^I'm ^causing ^any ^trouble. ^WUT?