Tumblr and Posterous both provide users with the ability to blog without all that much effort. But as similar as the two “light blogging” platforms are, Tumblr is the favorite among casual members of the blogosphere, according to traffic data from Quantcast.

Tumblr attracted around 42 million monthly visits to its hosted blog network globally (17 million of which come from the U.S.), while Posterous averages only around 7 million monthly visits on its hosted blogs, according to data from Quantcast.

But the main difference is in growth. Since April, Tumblr’s traffic has grown from around 2 million daily visits to 3.5 million, while Posterous has only seen a jump of about 50,000 users from 200,000 in April, according to Quantcast. But that has led to at least a few growing pains for Tumblr — namely with site stability, which Tumblr particularly struggles with as the site has frequently gone down.

Tumblr lets users create their own blogs from an existing library of templates, hosted for free by the company. The light blogging platform’s success has drawn comparisons to Twitter of 2008 and early 2009, as the company racks up big numbers of new bloggers each day. The New York-based company raised more than $10 million by April, with three rounds of funding led by Union Square Ventures and Spark Capital, while Posterous had raised $4.4 million in March in its first round, led by Redpoint Ventures.

WordPress, another competitor for Posterous and Tumblr, is still the clear favorite with around 255 million monthly visits globally to content on its Automattic hosted network. Automattic has raised about $30 million in venture funding, including a strategic investment from The New York Times Co., as well as backing from True Ventures, Polaris Ventures, and Radar Ventures.

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