Here's my tuppenceworth on Facebook.
I decided to make my SF novel available early last year having concluded that it's unsellable to a publisher. It's not a bad effort as far as I can tell, but seemingly unmarketable.
So I released it as a paperback through CreateSpace and as an e-book through Kindle.
For various reasons I did not try to push it actively. For one thing, having two babies in the house I find parenting leaves little time to think about that kind of thing.
Basically, I just wanted the book 'out there' so I could draw a mental line under the project.
However, a couple of things I did do to promote it were to set up a website and a Facebook page specifically for the book.
I then spent $50 on some Google advertising. This directed people to the website, where I have a plug-in in which people can 'like' the book on Facebook.
Checking the Google advertising stats, I seemed to get a reasonable number of click-throughs, and a reasonable number of complete strangers 'liked' the book.
I fully expected most of these to translate into sales. A grant total of 0 of them did.
So I am dubious about the value of web advertising a novel, and the worth of Facebook at all for this kind of thing. I suspect that all of these strangers 'liked' the book by accident.
In all I have sold about 20 copies of the novel in the available formats. Of these, maybe 7 are to total strangers.
Although my expectations are low, so to sell even 1 to a complete stranger is gratifying.