App vs. Web access
I, too, am unclear on App use versus "Web services" uses. The story says "Google was the top overall web service," and I'm assuming this is the search engine. Google is there as the default search engine on Android devices or it's accessed by default choice, bookmark or URL on other devices. So can you compare Google use to app use? Not sure it's Apples to Apples.
If I think about my own use of Twitter, it's heavily to comment on things on the Web, which lends itself to more of a browsing device like a tablet.