Exactly a week ago, we reviewed Ngmoco’s latest iPhone game, We Rule. Keeping in mind that Ngmoco made its name with quirky titles like Rolando, we marveled at how closely We Rule’s kingdom-building mechanics resemble the farming and time-management games that have vaulted Facebook developers like Zynga to prominence.

Only released for five days at the time, We Rule had already hit number 15 in the App Store’s free game rankings. Today, two days shy of its two-week anniversary, We Rule has become the fourth most downloaded free game, behind only three puzzle games.

Even better for Ngmoco, We Rule is number 12 in the top-grossing list, well above those three more-downloaded free games (though this last ranking seems to be somewhat fluid, as it also recently showed up at both higher and lower positions).

We checked in with Jason Oberfest, Ngmoco’s VP of social applications, for more stats. He tells us that We Rule now gets well over two million user sessions per day, with an “extremely” high rate of repeat sessions among players.

Although Oberfest isn’t sharing much more on We Rule’s specific stats, he did tell us that all of Ngmoco’s in-house, free to play games combined get 20 million minutes of play time a day. Based on that figure, it’s probably fair to assume that We Rule is already accounting for well over 10 percent of Ngmoco’s daily traffic.

Something even more interesting to consider: when we covered Ngmoco’s latest round of funding on February 23rd, the company told us that its free-to-play games had been downloaded 12.5 million times. Today, that figure is up to 30 million.

Since Ngmoco’s next highest-ranking game in the free category, Eliminate Boost, is currently far away at number 51, we can probably also assume that We Rule has been downloaded quite a lot, perhaps even over 10 million times. If so, it’s doing better than FarmVille did in its early days — although with fewer iPhones extant than Facebook users, it  would currently be impossible for We Rule to reach FarmVille’s 80 million monthly active users.

What is making We Rule so successful? Although quite similar to FarmVille-style games, it’s also fair to say that We Rule has its own distinct character, one that takes advantage of the iPhone’s unique form factor and — compared to Facebook — strong graphical capabilities.

Within your little kingdom, there’s just enough motion to make the scene seem alive, but not so much as to require a lot of attention. Harvesting, planting, collecting taxes, erecting new buildings — everything is accomplished with a few easy taps. From the level progression to the whimsical clouds floating across the sky, everything in the game is impeccably designed (ignoring some early server problems).

But although We Rule is more attractive and full-featured than your average Facebook game, our summary judgment would still be that Ngmoco’s big accomplishment has been porting the mechanics that make Facebook games successful (not to mention addictive, like withering crops) to the iPhone. And in the players, it’s producing a similar level of addiction.

As for We Rule’s earning ability, we’ve got no idea how much it’s making Ngmoco, but it’s worth noting that the title is one of only two free apps in the top 25 grossing category in the App Store.

And here’s where our final interesting detail pops up. The other free app within the top 25 grossing is Zombie Farm, by Playforge. This game is basically FarmVille, with zombies.

Zombie Farm appears to have been downloaded fewer times than We Rule, but has so far earned more — perhaps because players can’t really progress past level 10 without paying Playforge something. In We Rule, it’s entirely possible to never pay a dime.

Yet it’s notable that both of the iPhone’s top-earning free games use Facebook-style time management techniques, with farming featuring prominently. As more iPhone users get into games and iPads begin shipping to millions more people, it’s entirely likely that we’ll see Apple’s products create a new Zynga.