It's true. When you're agreeing with an author of a blog post, it just seems kind of silly to take the time to write out a comment that simply states: "I agree." So instead, the agreement or the liking of the post stays off-record. This means that even on a more universally well thought of post, the comments will often be the domain of the discordant few. This doesn't give a fair representation of the thoughts and reactions of the readers and, therefore, doesn't provide any true insight into the way the issue is perceived. It could be that 90% of the readers think the author is correct in their opinion, but only the 10% who feel differently have made their voices heard.
Lunar Calendar 2012