New data from comScore this week showed more than half of U.S. mobile users downloaded apps during the three months ending in May. Increased app usage globally will help increase spending on in-app advertising from $2.4 billion this year to $7.1 billion by 2015, according to a new forecast by Juniper Research. Besides a growing app audience, wider use of rich media in ads will also contribute to higher in-app ad spending. The report says rich media offers the opportunity for marketers to experiment with different features -- from mapping nearby stores to click-to-call to video to boost interaction rates. Juniper pointed to campaigns brands run in social TV apps like Shazam, which are tightly integrated with programming and…