I wrote The Kubic Kat as a standalone SciFi novella, and decide that it would be best to give this way free, in the hope that it would garner traffic to my start-up website, and interest in escaping surveillance. (http://www.tribalcontact.com)
I put it on Hacker news, but it sank without a trace, and yielded 0 comments, and very few downloads. Facebook, twitter, and Google+ have made up 30% of downloads, and Smashwords (https://www.smashwords.com/books/view/392425) most of the remainder. As long as you are on the front page you get good traction, but discoverability is poor.
Indeed, this is the biggest issue I know of. If you can put the book in front of people you can get good metrics (30% of all views led to downloads). But finding a way to get those views is very hard.