FORTUNE – Zynga’s long reign over Facebook may be coming to an end. The once-hot social game maker risks being eclipsed by the dramatic rise of a competitor, the aptly named King.com.

Figures from AppData show that King.com’s hit game Candy Crush Saga has overtaken Zynga’s (ZNGA) Farmville 2 to become the top game on Facebook (FB), with 9.7 million daily players compared to Farmville’s 8.8 million. The Europe-based King.com currently has 365 employees with offices in London, San Francisco and across Europe.

While Candy Crush Saga might lead the daily user figures, Farmville 2 is still ahead when it comes to monthly active users, with 41.8 million to Candy Crush Saga’s 25.8 million. Still, the growth has been remarkable considering King.com only launched its first game on Facebook in April 2011, even though the company has been around since 2003. Candy Crush Saga is even more recent, launching on Facebook in April last year.

“In less than 18 months King.com has become the second largest game developer on the Facebook platform with 70 million monthly active users,” said Julien Codorniou, head of European gaming partnerships at Facebook in a press release. “Only a few technology companies in the world have reached so many people so quickly.”

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King.com CEO and co-founder Riccardo Zacconi says much of the growth came from the games’ growing availability across multiple platforms and the synchronization feature, where a game may be started on one device such as a tablet and finished on another, like a smartphone or traditional desktop. “The transition to moving our popular games to mobile and making the playability both seamless and synchronized across multiple platforms has generated immediate interest from casual games players,” said Zacconi in a statement. “We’ve seen tremendous growth across both social and mobile games charts.”

Compared to Farmville, Candy Crush Saga is based on an extremely simple concept. It falls under the “tile-matching” game category: users win points for lining up three candies of the same color next to each other so that they disappear. Most of King.com’s games fall into this category of casual games, described as titles that are easy to learn, don’t take a lot of time to complete but can be difficult to master.

While traditional gamers have long looked down on tile-matching games for being too simple and not intellectually stimulating, the category is making a comeback along with the rise of smartphones and tablets. “Eighty percent of people in the U.K. play games on their phone regularly, what they’re looking for is convenience,” says Alex Dale, chief marketing officer at King.com. “They want something they can play in between breaks at work, coming home on the train or while relaxing on a sofa.”

IDC gaming analyst Lewis Ward says typical resource management games like Zynga’s Farmville, Cityville and Mafia Wars don’t always translate well to a mobile format, since they were developed for the PC and initially written in Adobe’s (ADBE) Flash, which isn’t supported on the iPhone. “For games to work on mobile, they have to be smaller, the length of play has to be shorter and the user interface has to be more intuitive,” says Ward. Given that about 60% of Facebook’s 1 billion users access it through mobile, it’s not a boat any developer would want to miss.

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Another part of King.com’s growth has been its successful adaptation of popular games across different platforms. The total number of gameplays across King.com’s network has increased to 5 billion a month, up from 1 billion a year ago. The King.com website hosts over 150 games but only the best performing ones are developed for Facebook. From there, the most popular are developed for mobile devices. King.com currently has 13 games on Facebook, out of which only the top performing Candy Crush and Bubble Witch Sagas have made it to Apple’s (AAPL) iOS and Google’s (GOOG) Android.

Dale says the approach allows developers to give the game the best chance to become a hit on every platform. “It enables us to understand what works and also allows us to mitigate the risk of business when re-developing,” he says. Another key component of successfully translating a game for Facebook is to “put it in a social wrapper.” This means enabling users to play with friends, either collaborating or competing against one another while sharing their stats and scores.

The privately held King.com has to date received $43 million in venture funding. Investors include Apax partners and Index Ventures. The company has been profitable since 2005 and currently has three different revenue streams. Along with display advertising, the games make money on “micro transactions” where players can pay small amounts of money to advance to the next level of a game or purchase a “charm” that gives them an extra life or bonus ability for a game. There are also tournament fees for more advanced gamers who pay to play against each other and win cash prizes.

The most lucrative advertising strategy might just be King.com’s “incentivized video ads” where players have the option of either making a purchase for an extra life or access to a new level or they can sit through a video ad for a sponsor. “Any company that can do it would probably prefer it to a banner ad,” says Ward. “The revenue per impressions for these kinds of ads is far better.” MediaMind recently reported that consumers are 27 times more likely to click-through online video ads than standard banners.

The rise of King.com has been rapid but there’s still reason to be circumspect, says Brian Blau, gaming analyst at Gartner. “When games become popular, they tend to take off very quickly,” says Blau. “King.com has the talent and the resources but it’s very hard to say whether they’ll continue to be successful given the fickle nature of the gaming industry.”