Do http://www.niemanlab.org:

While the news industry looks at smartphone and tablet apps as a chance to build more engaged readers — and maybe even get people to pay! — for those of us who watch the business, apps are frustratingly opaque.

With sites on the open web, there are any number of ways for outsiders to estimate the size of audience a news outlet is reaching. They’re imperfect, but they’re out there. But for, say, apps sold in Apple’s App Store, the only data available is pretty poor. For top sellers, you can see where they rank among other free or paid apps, but those numbers flit around from day to day. The number ofratings or reviews tell you something, but not a lot.

Íntegra

Decline, plateau, decline