I have experienced lately that people are leaving less comments on blogs. One of the reasons might be that social media like Facebook and Twitter has replaced the old fashioned and time consuming communication platforms like comments.
The power of Facebook's 'like' or Twitter 'retweet' is, that it is only a click. Nothing else. Majority of blogs and websites already have the Facebook like button inserted and by clicking it, you express both your positive feelings (i like, I agree) and you share the given content within your network.
Of course content-wise a comment is always more sophisticated. Things are not black and white. Maybe I like a certain content but I do not fully agree with it or I have some remarks, additions or questions to the topic. On Flickr (which is one of the largest photo sharing site) I have recognized that many people were irritated by the badges, group icons that other users have left under their photos. Some even claimed that "if you did not have a word to say about my photo, please keep your colorful badges". So, real words are always more respectful than pure likes or retweets.
But commenting is time consuming. First of all, writing a sentence makes you type plus many sites are requesting captcha or registration prior to send a comment. Most of us are using the internet in a speedy way. Only a handful people are online for one site at any one time, visiting a few communities and spending the time just with them. Those users are making a sort of intranet from internet for themselves, closing out the enormously rich online sources.
Others are rather grabbing as many information as they can. Since laptops and computers became quite powerful, it is possible to have 8-10 windows or tabs open at the same time. Reading an article in one, writing e-mails in an other one, surfing among photoblogs, watching a movie in the corner or listening to music in the background. For this style of usage commenting is too slow. Pushing the like button totally fits this multitasking, overcrowded style. Dealing with comments makes you to stop anything else.
That's why I think commenting has become outfashioned.