Could your problem be that you're pushing the wrong product? If people don't find your product useful/exciting/etc, there's no reason to expect any amount of coverage to make an impact on your traffic.
I think they probably consider buying a single track from an album and buying a 'digital single' (a small release with 1 or more tracks) two different things.
I doubt the former case is considered a 'single' in the traditional sense of the word in the music industry.
Convert is listed as #2 in the overall chart; that link is talking about an app in the top 30 in the social networking chart. Two different charts (with the overall chart being quite a bit more competitive) explains the differences.
What I think a lot of people tend to forget is that Verizon and AT&T have based their networks on differing technology (GSM vs. CDMA).
Apple seems to like keeping their product lineups relatively simple. GSM tends to be the worldwide standard. Fracturing their product lineup for (generally speaking) one market doesn't sound like the sort of thing Apple would do. I imagine if Verizon was the major GSM network in the U.S.A., we'd see Verizon + Apple instead of AT&T + Apple.