Download volumes for the top free iPhone titles in the US increased during May 2012, while the cost to acquire loyal users fell, according to research from app marketing company Fiksu.

According to the company’s App Store Competitive Index, aggregate daily download volumes for the top 200 free iPhone apps in the US increased by 7.3 percent to 4.54 million, from 4.23 million in April 2012. The Cost per Loyal User Index fell by 13.7 percent in May to US$1.26, from US$1.46 in April.

Micah Adler, Fiksu’s CEO, said that May saw marketers gearing-up for the summer, which has traditionally presented an increased opportunity for app promotion.

However: “amid concern around Apple's speculated deprecation of UDID, compounded by a host of confusing alternative UDID-less tracking technologies, we observed inertia. It appears that brands' caution may have contributed to the dip in acquisition costs.”

Fiksu previously noted a downward trend in app download volumes, as no events took place to drive downloads – such as a new iOS device launch or holiday sales period.