Many of iOS’ top-grossing developers have built out scores of concepts and may have more than a dozen apps or extensions of a single franchise under their name. But San Mateo’s The Playforge has taken a distinct, very focused approach.

The bootstrapped company has put almost all of its attention on an endearing, little Farmville-meets-Plants Vs. Zombies mashup called Zombie Farm.

Released last February as a hobby project by co-founder Vince McDonnell, the game has become firmly ensconced among the top 10 highest-grossing apps on the store for an entire year and has seen roughly 13 million downloads.

“We want to focus on quality titles,” said McDonnell, who came from the console gaming world before switching to mobile. “We want to make sure games are actually fun before releasing them. And we want to make sure that they thrive.”

The company is looking at releasing at least one other title this year, but for the most part it continues to improve upon Zombie Farm. An iPad version of the game is due out in the next two weeks to a month.

Zombie Farm has the familiar feel of Farmville; the opening part of the game looks almost identical to the fabled Zynga title. But Playforge’s game comes with the irreverent addition of zombies who can can be used to battle neighbors. The brain-eating, drooling zombie theme unsurprisingly makes the game skew a little more male than most collaborative farming games. The paid currency, which players buy via an in-app purchase, is actually brains.

Unlike many other top mobile apps, Zombie Farm actually has a strong Facebook integration. The game prompts the user within the first couple of minutes to connect to the Facebook platform. Playforge says its Facebook usage rate is within line of what other strongly-integrated mobile developers see, with 10 to 30 percent of active users electing to log in with Facebook. On AppData, our tracking service for the top apps and developers on Facebook, the game has just over 516,000 monthly active users on Facebook and nearly 75,000 daily actives.

“When the game first came out, I assumed my primary audience was going to learn about the game through Facebook,” said McDonnell, even though that hasn’t necessarily been the case. “I sort of left it that way.”

On iOS, Zombie Farm has thrived where Zynga’s Farmville has given a mediocre performance considering the company’s financial resources and existing base of players on the Facebook platform. Farmville temporarily peaked at #2 on the highest-grossing list last week on the back of a 40 percent off Farm Cash sale before falling back down to #47 today.

“Zynga’s Farmville app was really a companion and not really meant for iPhone consumption,” McDonnell said. “The experience was a shrunken down version of the Facebook game.”

Playforge gives Zombie Farm extra life by using biweekly updates to refresh the content. A recent one brought a Spring Break theme where zombies have to fight partying lobsters off their beach. Updates can re-engage as much as 25 percent of the player base, who decide to open the app after downloading the update. The company is observing the Android ecosystem closely and looking to see whether in-app billing or the Amazon appstore pays off for developers before launching titles on the platform.