Keep in mind that also have a superior product to just about every image host that came before them (other than Flickr), they are the de-facto favored image host for Reddit, and to top it off have their own social networking features with non-trivial market adoption.
With that many eyeballs on your site it's no surprise that even their subtle advertising style [1] is profitable. I'd personally much rather see sponsored posts everywhere than pandering direct marketing and spying/tracking.
[1]: http://www.imgurads.com/
