The number of active subscribers for Blizzard Entertainment’s most popular online game, World of Warcraft, fell in the second quarter of 2011, the second decline in a row.

World of Warcraft is one of the most dominant online games in the industry. It’s enjoyed that position since it launched in 2004, peaking at 12 million active subscribers. That number has since fallen for two straight quarters, a sign that the game might now be showing its age. It’s given other online games, like Trion Worlds’ Rift, an opportunity to step in and try to steal market share from the massive multiplayer online supergiant.

It’s also been a huge source of revenue for Activision-Blizzard. World of Warcraft charges players $15 a month to access the game. The company said digital revenues from properties such as World of Warcraft hit a new record, showing a 27 percent increase from the same quarter a year ago. Digital revenues now account for 37 percent of the company’s total revenues.

“It’s normal to see some declines, the team is currently working on our largest content update since Cataclysm, which will hit later this year,” Blizzard Entertainment chief executive Mike Morhaime said.

The company is working to open the game in new markets and acquire new players, Morhaime said. The game’s latest expansion pack, World of Warcraft: Cataclysm, launched in China last month. The company also made World of Warcraft free to play for the first 20 levels of the game in order to attract new subscribers.

“We have seen an increase in new account creations as a result of that,” Morhaime said. “It’s still too early to tell on conversions to subscribership, but I believe that is an important direction for us.”

Blizzard Entertainment is also working on another massive online game that it has not announced yet. The company is diverting more revenue to development of that title, Morhaime said.