Subscriptions to World of Warcraft have shot back up on the strength of the Warlords of Draenor launch.

There are now more than 10 million active subscriptions to the MMO. In June, the subscriber base had slipped to 6.8 million.

Blizzard has sold through more than 3.3 million copies of Warlords of Draenor since its release last week.

"We're thrilled that so many players jumped in to play Warlords of Draenor on day one and are having a great time with the new content," said CEO Mike Morhaime.

"We poured a lot of hard work and passion into this expansion, and we appreciate all the kind words from players and the overwhelming support they continue to show for World of Warcraft."

The success of the Warlords of Draenor appeared catch Blizzard off guard. Server stability over the launch weekend was uneven.

Blizzard apologised for the server issues, and announced plans to make it up to players with five free days of game time for all active subscriptions created on or before November 15, 2014, AEDT.

“The first two days of the expansion were not a great experience, with many of you facing high queues or significant gameplay problems,” wrote Blizzard's J. Allen Brack on Battle.net. “We worked around the clock to tackle the demand issues and technical challenges, and fortunately things started looking better on Saturday.”