The data I've seen so far indicates that that is the case, although matgb makes a good point about it probably being more about the opposition than anything inherent in the left. I mean, obviously not all bloggers are leftist, which is why I said "tend" rather than "are." I'm sure I'd only have to Google for a couple of minutes to find conservative (American or actual) writers or a nationalist or xenophobic blog.

And there is an inherent social bias in internet use, even today. You need to have a computer, access to the internet, and the skillset to be able to blog, not to mention needing the leisure time to participate in the dialogue.