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StumbleUpon - the social exploration site and Digg rival - is advertising on Digg.com, spotted Flickr user Michael Pinto.
It gets better: StumbleUpon isn't looking for users, but rather promoting StumbleUpon Advertising. StumbleUpon stealing advertisers from Digg using Digg Ads? Amusing, but not necessarily intentional: StumbleUpon could have made an ad buy across a number of sites. Regardless, it's time to enable the competitive ad filter, Digg!
Mashable on Digg