Another very important metric, probably more important than raw traffic is number of users and number of posts to each service. You can get more creative in number of posts per blog per month, recency data (e.g. X% of all users have blogged at least once in the past 90 days) and get an even better picture.

Without this kind of context, raw traffic is kinda meaningless. At least the quantcast numbers for both are directly measured and not "estimated".