Google wrote an interesting post about commenting on websites/blogs whatever. Here is a little bit of the post:
Read more here:Hard facts about comment spamIt has probably happened to you: you're reading articles or watching videos on the web, and you come across some unrelated, gibberish comments. You may wonder what this is all about. Some webmasters abuse other sites by exploiting their comment fields, posting tons of links that point back to the poster's site in an attempt to boost their site's ranking. Others might tweak this approach a bit by posting a generic comment (like "Nice site!") with a commercial user name linking to their site.
Just like Will said, it isn't that good anymore to buy blog commenting etc. It might help a little bit, but that wasted time and/or money can be invested in better things