I'd love to read an article about "Growth Hacking Gone Good"...because I'm sure there are a few good cases to follow, in situations that were more difficult to get growth than Rap genius.

The first thing that comes to mind is Imgur. I've only read about the site's growth casually, but what I remember as a user was that one day, someone announced the service to Redditors, people loved it, I liked it, but assumed it was a small minnow in an ocean that include Flickr, Picasa, Tinypic, Twitter-friendly services like yFrog, and so forth. Even more problematic was that the feature that made Imgur likeable -- hotlinking hosted images -- was basically impossible to attach user-retaining measures/loyalty/SEO-hacks, etc, all the while sucking up massive bandwidth.

Edit: also worth noting: not only is Imgur not a place to (easily) search and revisit, as a domain, it has the same copyright-infringing/duplicative-content risk that RapGenius does too.

And yet Imgur is now a behemoth, by not jilting its main feature for easy profit...I'm still wondering..how the hell did they do that? I pay attention to imgur because I'm a geek who often looks at URLs. Even among redditors I must be a rare case, but my perception is that Imgur did things so right and so well that, now they're close to becoming a near-essential service of the Internet