Downloads from Apple's App Store declined in April as the cost of landing loyal users increased, according to Fisku, an app ranking firm located in Boston.

Daily downloads of the top 200 free apps in Apple's App Store dropped by 4.9 percent to 4.23 million, down from 4.45 million in March, Fiscus reported Tuesday. The decline comes a month after Apple celebrated breaking the 25 billion mark in app downloads at the store.

At the same time, the cost to app developers to corral loyal users climbed 12.3 percent in April to $1.46, from $1.30 in March.

According to Fiscus CEO Micah Alder, the April decline wasn't surprising. "April’s continued downward trajectory of app downloads was expected, as no major events sparked discovery during the month," he said in a statement.

“Additionally," he added, "app marketers continued to scale back their use of robotic install tactics in response to Apple’s policy on third-party marketing services."

In February, Apple cautioned developers they could get kicked off the iOS gravy train if they used third-party marketers to manipulate their app's ranking at the App Store. Apple's action was aimed at the deployment of "bot farms" and "water armies" to propel an app into the Top 25 rankings by using paid mobs of users to download the software and give it positive reviews.

As downloads declined in April, spending on marketing began to rise, Alder noted. "[W]e observed more aggressive advertising spending as marketers moved out of the first quarter lull and began to seek new opportunities to compete for rank especially in the dynamic social networking and games categories, which experienced volatility during the month," he said.

Heating up developers' interest in social networking apps was Facebook's eye-popping purchase of Instagram for a billion dollars, the Fiscus report noted.

"Additionally, a number of new gaming apps hit the market in April, driving fiercer competition in the popular games category and increased interest from the developer community at large," it added.