Although I haven't been checking in as frequently here as a few months ago, I would say that there is definitely a "MobileRead effect" that can help an author. (My strong suspicion is that I have also benefited from those "word-of-mouth viral promotions" Neil mentioned.)
That said, there is also a sort of saturation level that occurs
regardless of venue, beyond which point your downloads are likely to become self-sustaining. The closer you come to the top of the chart, the more likely people will be to check out your material, simply because it
is near the top of the chart. This isn't necessarily a bad thing, because it means that some folks you wouldn't normally consider part of your target demographic might wind up giving your story a chance... and they might even like it.
Of course, the hope is that those same readers will then keep an eye out for your name, and seek out other, new stories as they become available.
- M.