They say the players are going to "free to play" games.... unfortunately it looks like RuneScape isn't one of them!
The company said 14% of users left the game between January and March - a fall of 1.3 million.
The majority of lost users were in the East, despite the company making efforts to appeal to those markets, with recent upgrades.
Activision told investors to expect subscriber figures to dip further.
Shares in the publisher fell by about 5% on Wednesday, following the news.
The California-based company blamed the rise of free-to-play games.
Uncertainty around new consoles due from Microsoft and Sony - and the "very slow start" of Nintendo's newest offering, the Wii U - had contributed to what will be a challenging 2013 for the company as a whole, chief executive Bobby Kotick said.
Martial arts pandas
He told investors on Wednesday: "It's important to note that the nature of online games has changed, and with the environment becoming far more competitive.
"To address this, we're working to release new content more frequently to keep our players engaged longer and make it easier for lapsed players to come back into the game."
Despite the user dip, the company said World of Warcraft still had 8.3 million players - making it the most popular subscriber-based online game in the world.
"Warcraft's exposure to ongoing developments in online gaming keeps growing," said Steve Bailey, a senior analyst at IHS Screen Digest.
"It is coming under pressure on several fronts - subscription-based online gaming is losing ground to micro-transactions and hybrid-monetisation models."
World of Warcraft's subscriber numbers were as high as 12 million in 2010.
In October 2012, 10 million gamers were signed up - a figure boosted by the release of Mists of Pandaria, an expansion pack that sold 2.7 million copies in its first week.
That title, which featured a new "continent" on the game inhabited by martial-arts-skilled pandas, was targeted at the Asian market.
The push reaped initial rewards - user numbers swelled to more than one million in China - but the figures made public on Wednesday suggested the spike was short-lived.
"This is no surprise," Mr Bailey added.
"China is a very advanced and fiercely competitive market for online gaming, that, as well as offering greater direct competition to World of Warcraft, is also experiencing its own shifts in terms of business models and devices."
Beyond World of Warcraft, Activision anticipated further difficulties in 2013.
Skylanders, the company's series that allows players to use real-life toy figures to interact with the game using near-field communications technology, has been very well received by critics and gamers alike.
But it will face stern new competition from a major player - Disney.
The media corporation plans a similar title with add-on accessories, Disney Infinity, to be released in the summer.
Enhanced interactive features and the benefit of a wealth of highly recognisable and well-loved characters could give Disney an edge, some games enthusiasts predict.
But Activision is determined to remain competitive and already has an eye on the Christmas market.
"The secret to Skylanders' success from the very beginning has been to continually surprise and delight our fans with meaningful innovation and superior gameplay," said Eric Hirshberg, chief executive of Activision's publishing division.
"This [autumn], we are pushing the genre forward once again with Skylanders Swap Force. Swap Force introduces an all new play pattern - dynamic swapability."