I don't usually rant on this blog, or go at length into my pet peeves, but there's one topic I really feel should be aired for the potential to make the world a little better.
It has happened to me more than once that I was besieged with reading requests, review requests, writing help requests, and I carved some time out of my busy day to do it, reading other people's work and providing helpful feedback.
And when I was done, I received no acknowledgment - no "thank you", no "I appreciate it". Nothing.
I'm not a vain person or an egomaniac, but we're talking common courtesy here. The way I see it, if you're asking someone for a favor - which a review or feedback basically is - you thank them. It's as simple as that. To ask a person to put in their time and effort when they don't owe you anything, and then omit sending a "thank you" message is not just rude, but also incredibly short-sighted.
A book reviewer can be a lot more than a one-time contact. They can become someone who follows your social media, signs up for your newsletters, recommends your books to friends and reviews your next book without being asked. It is, in short, another option of expanding your social network, but it is unlikely to happen if you fail to acknowledge the goodwill of someone who responded to your review request.
Bottom line: thank people. Always be polite and courteous, whatever the circumstances. You will never lose anything, and might gain a whole lot, by doing so.