The main problem is where to leave comments. Many people say that leaving piles comments, but Google does not register those comments, why?
Because most of these blogs, squidoo or huba page is not ranked or have a poor ranking, and Google does not see them.
Comments should leave on well ranked blogs, squidoo huba pages.
1. You Tube's blog (yes You Tube has a blog)
2. Twitter blog
3. Official Google blog
4. Stumble Upon officially blog
On official social networks blogs, and your freiands profiles, pictures, groups.
All these blogs are very well ranked on Google, but do not leave no sense comments, leave comments that are related to the post.
On You Tube, leave comments on the post with similar topic as you last video, it will increase the range of your video on You Tube and Google.
Google officially on the blog can not leave a comment. On the right side find 'Newest Google blogs, on some you will be able to leave comment, and on some not.
On the right side find "What We're Reading'', you'll find a ton of blogs, they are all blogs that are very well ranked on Google, and Google will value that more links.
Answer to Adi's comment:
If you look on the official Google blog in the category ''What We're Reading'', there are many WordPress blogs, and all are very well ranked.