I have a feeling that the SEO battle for their keywords is about to get a lot more fierce though. I doubt they've solidified their spot in Google yet, so any tweak on Google's part or a blackhat SEO could seriously set them back.
a) Google loves unique content. Unique content on old-world lyrics websites consists of comments buried under the fold. Whereas these guys have pages and subpages full of it.
b) They're going to get a bajillion inbound links because their content is interesting to link to (as opposed to standard lyrics sites). Which will rank them higher than their competitors for the same queries.
I see a lot of comparisons with Yelp. I still type in business names with "Yelp" appended into google because I want a specific result. Wouldn't be surprised if they're getting the same sort of queries already since what they have is so much better than any lyrics site.
Side note: I just opened AZ lyrics and some flash abomination nearly crashed my browser.
This is true today, and RapGenius has a great linking feature that lets you link to a single line explanation in the lyrics (helpful so the recipient doesn't have to scroll through and find the line of interest).