This activity is courtesy of Sue Waters who says:
If readers have made time to comment on your posts, you should always make sure to respond back (ideally to each reader) in the comments on that post. This is very important for building your blog's community as it demonstrates that you value your readers and their input.
Commenting back also increases community interaction. Look at how Lee is interacting with her readers on “What Posts Stimulate Readers To Comment?” and while you are there make sure you leave your thoughts on what makes readers comment.
If you blog about this, be sure to tag it with "comment08."