Facebook’s top developers say the company’s payments infrastructure and virtual currency Credits is converting fewer paying users than they had hoped a year ago.

Facebook made it mandatory for developers to use its payments platform in canvas games in July. That meant developers on the platform had to start handing over a 30 percent revenue share to the company, mirroring a similar split on Apple’s iOS. The hope was that a single, universal currency would make it more frictionless for users to start paying for virtual goods.

“We thought that conversions would go up and be around 15 or 20 percent,” said Kevin Chou, the chief executive of Kabam, a social gaming company that targets a more hardcore demographic, at the Inside Social Apps conference in San Francisco. “But it turned out to be around 5 to 10 percent, meaning that we’re taking a 20 percent net tax.”

For comparison, Facebook’s biggest developer, Zynga, revealed in its prospectus that it had 3.4 million unique payers during the third quarter of last year. That’s out of 152 monthly unique users in the same time period, suggesting a 2.2 percent conversion rate.

Anil Dharni, who co-founded Funzio, which has had hits on iOS and Facebook like Crime City, said the move to Credits ended up being roughly even for the company.

“Facebook credits is a wash for us,” he said. “It increased the conversion rate but we actually saw a gradual decrease in average revenue per paying user. It’s hard to know why.” Funzio has since moved its focus to iOS, where it has launched Crime City and Modern War, both titles that reached the top of the grossing charts.

Continue reading on our sister site, Inside Social Games.