Short answer: Kind of
Full answer: No, you don’t, but it helps.
Here’s why: You have to have a place to write up how you’re doing on your goals that you can link to each week on the Check-in Linky so that your fellow ROWers can come visit and leave you encouragement. Tweets, Facebook updates (on your own wall), and the like don’t work well for this purpose. We do have a Facebook group that you might find a more welcome place to connect with our community. You can find that here: A Round of Words in 80 Days Facebook
That being said, nobody said you have to have a popular blog or even one that’s going to show up in search engines. A lot of people resist the notion of having one at all because they don’t want to keep it up. I’ll leave it to Kristen Lamb and her We Are Not Alone: A Writer’s Guide To Social Media to convince you of why it’s never too early to start your platform and why you, as a writer today need a blog (it’s an awesome instructive book and you should read it). I’ll just say that for the purposes of ROW80, there is nothing in the world wrong with starting a free blog at WordPress or Blogger for the express purpose of providing your updates.