App-install ads are a major success story in marketing these days. Facebook claims it will be responsible for 4 billion app downloads by 2017, representing up to 20% of its ad revenue. Google recently claimed its app-install ads prompted 2 billion downloads. The hype is real. That's about one app download for every human, which makes this advertising tactic among the most effective that mobile has ever seen. But do these alarmingly large numbers mean that native apps are a winning strategy for brands?