One of the major components of the Internet is sharing. Thousands, or millions, of people are sharing their information with others, usually for free (in one way or another).
Don’t you think that the least we can do is take a moment after using someone’s information and write them a note thanking them for sharing?
Many sites have a space at the bottom of articles or blog posts to write a comment. It takes just a moment and I can tell you from being on the other side that it is extremely appreciated (that’s an understatement). Because otherwise, most of the time it sort of feels like you’re talking to a void.