For each blogging service, the uptime of the homepage and four individual blogs was monitored. Tests were performed once a minute over a period of two months, between October 15 and December 15, from multiple locations in North America and Europe.
As you can see, the winner is clearly Google's Blogger as it had no downtime whatsoever. WordPress.com came in second with very little downtime. Typepad also did well, while Posterous was given a passing grade. Tumblr was the only service in the test that truly failed.
Tumblr has had a rough time with their stability lately; some Tumblr blogs were down for a total of more than two days during the two-month period of the survey. Tumblr also had the single longest outage by far, where its blogs were unavailable for almost 24 hours earlier this month.