The answer may surprise you. According to this Wired article: Thinking Out Loud – How successful networks nurture good ideas:
Every day we collectively produce … over 1 milion blog posts and 2 million blog comments on WordPress
Not bad at all! (Disclaimer: The article does not clarify how the author computed that number or whether this only includes WP blogs on WordPress.com so of course there is no way to know the accuracy if this number).