If someone takes the time to comment on your blog or respond to one of your tweets, respond. Thank them. These types of comments are as close as many of us will come to fan mail.
The real use of a viral marketing campaign is to build or strengthen the relationship between you, your product and your customers. For writers, your customers are people who read what you write.
After a long day of working at your day job, writing, editing or whatever else keeps you busy, you may not FEEL like responding. Tough. Suck it up. If you want to be a professional, then treat your fans with respect.
You can even turn this into another viral marketing campain. Consider responding to comments by video. You could also consider responding to emails/comments like Joe Konrath does (Joe Answers Your Questions). This turns responding to comments into a media event. Very smart.
If you are truly too busy to respond to comments, consider shifting your priorities. For any small business owner (which is what you are), ensuring your customers are happy is more important than anything.
BOOKS BY M JOSEPH MURPHY:
Amazon: M Joseph Murphy on Amazon: Paperback and ebook
Smashwords: M Joseph Murphy Author Page on Smashwords
Kobo: M Joseph Murphy Books on Kobo