blogsquatter: a person (with or without his/her own blog) who uses somebody else’s blog to express his/her opinions. Can often be found writing long entries in the comment section of a popular blog. Could also be called a siteparasite.
Thanks to A.J., a self-proclaimed blogsquatter (who makes very interesting comments though!) who came up with the term in an e-mail exchange.
The term blogsquatter can be found in a couple of places on the Web but it does not seem to be used with the above definition in mind.
See: Karen McCombas blog squatter
The Diddly Squatter Blog