This is an increasingly common dilemma faced by authors. I think the key is to not worry about it, because their is really not much you can do to stop it. It is also wrong to assume that these individuals are lost sales - there are no solid metrics supporting that.  

What you can conentrate on is creating a loyal fanbase that will gladly pay for your book because they enjoy your work and your pleasant disposition. Becoming the 'guy who rants about book thefts' will only make you a greater target. Just look at what happenned to Metallica after they went after napster. 

Kind Regards
H