Blogs you can rely on to keep your content online: Blogger, WordPress, Typepad...

Want to make sure that your loyal readers will always be able to keep their eyes on your online blog?

Web monitoring company Pingdom has tested the most-used blogging services to see how reliable they are over a period of two months.

The five most popular sites used for blogging included Google's Blogger, Wordpress.com, Typepad, Posterous and Tumblr.

"For each blogging service, we monitored the uptime of the homepage and four individual blogs, so we could see how the service as a whole performed," said Pingdom in a December 17 blog post.

"The winner was without a doubt Google's Blogger," reports Pingdom. "The Blogger blogs didn't have any downtime whatsoever during the two months we monitored them, followed by WordPress.com which had very little downtime."

The average downtime for popular blogging site Tumblr was 47.5 hours. Posterous had the second highest rate of downtime, 2.1 hours while rivals Typepad and Wordpress.com were almost always online with just 0.2 hours and 0.1 hours of downtime respectively.

While most of the blogs in the survey saw very little downtime Tumblr was comparatively unstable. Some of the site's blogs experienced a total downtime of more than two days in the two month period from October 15 to December 15.

Some of Tumblr's downtime can be attributed to its rapid growth. The site currently hosts more than 11 million blogs and sees in excess of 500 million page views each month.

"This rapid growth is bound to be a factor in the downtime we've seen," reports Pingdom.  whilst suggesting the most important factor is how the site will increase its capacity in the future to match its increased popularity.

The race for reliability was very close between Blogger, Wordpress.com and Typepad.

"Overall, with the obvious exception of Tumblr's current predicament, these blogging services are all quite reliable," says Pingdom whilst cautioning that "[t]here's of course more to choosing a blogging service than mere availability."

In terms of keeping blogs up and available, the winners' stand looks like this:

1. Blogger
2. WordPress.com
3. Typepad
4. Posterous
5. Tumblr