U.S. digital game sales grew to $841 million in June, up 4 percent compared to May but down 1 percent from a year ago, according to market researcher SuperData Research.

The data measures the full range of the game market, including boxed PC and console games, free-to-play games, and paid digital games. Sales slipped in June largely because of the weak boxed software market for PC and console games. That market was down 16.7 percent.

Social average revenue per paying user was $36.67, with total sector revenue at $133 million.

Overall free-to-play games grew 9 percent from a year ago to $489 million in the categories of social, mobile, and massively multiplayer online (MMO) games.

Monthly revenue for free-to-play MMO games was $205 million, more than double compared to a year ago.

Spending on in-game items among MMO subscribers was $24 in June, up from $17 in December. Revenue in mobile games was $147 million, up 1.7 percent from a year ago.

“As the number of monthly uniques that play social games further declines, reaching 89 million in June, the underlying metrics have started to improve,” SuperData said. The company said that social networking users continue to shift from Facebook to iOS and Android.

Social casino games continue to grow on Facebook, as the average revenue per paying user among social casino games is two to three times that of average social gamers. Leaders in the market such as DoubleDown Casino are generating around $500,000 in daily revenues.

The number of free-to-play MMO users hit 46 million in June, and the average spending per paying user grew to $27. Games like League of Legends from Riot Games are proving that gameplay mechanics and free-to-play monetization are sustainable in Western markets.

The paid MMO market was down slightly at $83 million, compared to $89 million in May. The sector has stabilized at about 6.3 million MMO subscribers.