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.
Your article dates from half a year ago and they still couldn't snuff imgur out with built-in upload facilities, suffice to say, as far as the users are concerned, imgur is the defacto beloved service. People on twitter are more likely to use twitter's img upload than redditors are to use their own.
