As writers, we all know promoting on social networking sites is a slippery slope, you don't want to over do it, but it's a necessary evil. But another aspect of this is, you really should respond to people who take the time to comment on your promo posts. I mean, unless you're at JK Rowling level of fan base, in which case okay, you can't respond to every one. But most of us aren't. When people comment on my facebook page that they liked one of my books, I always say thank you, or at the very least acknowledge their comment. I mean, a simple thumbs up will do! Because it could affect the way the readers think about you. At least it does for me.
Example? A couple weeks ago, on a promotions group I belong to, a self published author posted a promo - this author was giving away a free ebook, because she was about to put out the second volume, and wanted to get some attention to it. I love free stuff, so I downloaded it. I went to take a look, and found that unlike a lot of self published books by unknown authors, this was pretty damn good, engaging, and unique. So I took the time to comment on the post, and tell the author how I felt about it. I gave a compliment. I took the time to do that. What did I get back? Nothing. The author never acknowledged my comment. It wasn't that she never came back to the group, because a day later she was back, doing more promo. I noticed another couple of people commented on said post, and still she never acknowledged them. Not even the lazy-man's thumbs up.
The thing is, I was ready to PURCHASE this second book of hers when it came out, based on the free promo. But because she didn't acknowledge any of the comments readers made, complimenting her on her work, I decided FUCK IT. Is my one lack of sale going to make a huge difference? Probably not. But that's one sale she could have had, and doesn't. Just because she was a dick. So don't be a dick. You never know what effect it's going to have. If someone compliments your work, emails you, posts on your social networking page - and you're not completely bogged down with fanmail like the literary Justin Bieber? Acknowledge it. Or it could cost you.