Via Thursday Bram, communications agency Universal McCann recently conducted the third wave of their global study on social media usage. The results indicated, of course, a growing usage of all kinds of social media worldwide. Also, it notes that "blogs are a mainstream media worldwide and as a collective rival any traditional media" (emphasis mine). Sooner or later, it seems we'll have to be more specific when we say "mainstream media". :-)
You can see a complete slide show of results here. (Warning: It's very colorful, and people with a sensitivity to circles should not consume.) It should be useful for citing whenever a convincing intro to a SM research paper is needed.