DO contact the sites and ask them to take the content down:
After all, it takes a lot of work to write a novel and I don't blame anyone for being upset over their work being stolen from them. It's illegal and wrong.
DON'T rant in social media about it:
What many authors don't seem to realize is that by ranting about it over the internet, they're actually drawing attention to the problem. People who may have otherwise bought your book will be aware now that it's possible to find your book for free online. They might go searching for it.
I was a huge fan of Stephenie Meyer and the Twilight series. When she was writing Midnight Sun, I was waiting anxiously to receive it. Suddenly, there was an update on her site about how upset she was because the partial rough draft of her book had been leaked online. I asked everyone I knew if they had read this partial rough draft or had even seen it. All of them shook their heads. I'm pretty positive now that only a handful of people at most saw this and that her concerns were blown way out of proportion.
My friends and I all decided that we'd buy the book even if we could find it for free online, but she decided to stop writing it and post the partial manuscript on her site. I think she was trying to be passive aggressive or something. Punishing the fans (who didn't even do this) by refusing to write the rest. I downloaded it for free and she missed out on millions of dollars because she decided never to finish it.
I don't care for the Twilight series how I used to and if she finished her book now, I probably wouldn't purchase it. She missed out on me being a customer of hers because she let herself get too upset over her manuscript being leaked to a small amount of people online.
It wouldn't have made much of a difference and hardly anyone would have read it (who wants to read a partial novel anyway? I want the whole thing!) if she'd have just ignored it, but she decided to let it control her decisions.
It's okay for an author to be upset when someone steals their work. A lot of effort and tears were put into their creation. But to draw attention to the problem publicly is as counterproductive as ranting about a one-star rating on amazon. Most people won't notice if you don't draw attention to it.
DO try to educate your fans on the importance of supporting authors financially:
Some people only download this stuff because they assume that authors all write bestsellers and are wealthy. They don't think we'll feel the financial loss of it. If an author is honest with their customers about the amount of work they put in a novel and educate them on the true state of most author's finances, there are many (not all) who will change their mind and actually pay for the novels they downloaded illegally.
The key is to be positive and honest. Don't make it about yourself, make it about all writers everywhere and how they need this support. It keeps you from seeming greedy and selfish. Don't point out the fact that some of your novels might currently be on pirating sites. This keeps people from searching. Don't talk to readers as if they are the enemy trying to steal your work, but like they are friends. It'll make them think twice about it.
DON'T lose sleep over it:
In the end, if a book is popular enough it's going to get pirated sometimes regardless of what an author does.
Neil Gaiman's novels and comics have been pirated many times. He claims that in areas where more pirating happens that he sells more books. It's a type of free advertising. Everyone is not going to download your books, but the people that do might tell others how much they loved your book. Libraries have been giving people free reads before there was ever an internet. They'd purchase one book and then give it to hundreds to read for free, but in the end, instead of destroying authors it has helped them. In the same way, someone who pirates a book probably bought it and is letting a bunch of people read it for free as well.
I'm not saying pirating is okay. Many authors struggle for every sale they get, but don't automatically assume that because some people pirated a novel that sales for that novel are destroyed. If Neil Gaiman's theory is the truth, then it might actually help you in the end.
Either way, by letting it hurt you too much, you could let a pirate destroy your career by taking you down enough that it's hard to write. Don't let them have that kind of control.