Yes... they are. Although it also seems like the Humane Society of the United States is made up of a number of different organizations... some of which are 501(c)(3) corporations.

I found their 990 form (the tax form that non-profits have to fill out)... and... they ARE a non-profit. Here's the link to the 990s they make available to the public:

http://www.hsus.org/about_us/annual_reports_financial/annual_reports_financial_statements.html
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