They’ve done it again: with FrontierVille, a new mix of the farming and city building genres set in pioneer days, Zynga has added some 7.4 million monthly active users to its total. That’s in only 12 days since the game’s launch, with the most significant growth taking place over just the last four days.

Zynga became the world’s top social gaming company for a reason: it’s masterful at growing an audience. However, FrontierVille’s huge success also comes at an interesting time for the company. Last Friday, it hit a milestone of another sort, dipping below 200 million monthly active users for the first time since it passed the mark last November.

On Saturday and Sunday, Zynga recovered, inching back over 210 million MAU. But the company has shed users steadily for several months now, a fact that suggests its MAU could sink further yet.

In the past, we’ve also pointed out that Zynga’s daily active user count is more stable than MAU — an important point, since DAU is a more significant measurement of how many serious players a game has. But while Zynga’s DAU has held steady at times, it’s also declining over the long term; in February Zynga’s DAU peaked at over 70 million, while today it’s around 50 million.

Zynga is often the target of criticism within the gaming community. However, its success rate with players is not at fault here. The company has found hits in two thirds of its releases this year. The thing is that there have, in fact, been only three releases: FrontierVille, Treasure Isle (currently its third-largest property) and Poker Blitz, which never took off, but also wasn’t heavily promoted.

That’s a rate of one game every two months, assuming Zynga doesn’t release another before the end of June (we have no expectation that it will). By contrast, Zynga released at least one new game a month in 2009, including its mega-hit, FarmVille. With bimonthly releases like Treasure Isle, which topped out near 30 million MAU, Zynga’s traffic could still stabilize at over 100 million MAU; but Isle is just one data point.

At its new, slower pace, Zynga resembles the rest of the social gaming world. We’ve seen fewer releases from most of the major developers this year. Development is becoming a more rigorous, expensive process, and since Facebook’s notification changes, each release has a harder time becoming successful — meaning even more investment to ensure that each release does well.

While it’s possible that a game like FrontierVille could blow past Treasure Isle to become the new FarmVille, capturing more than 60 million players and bringing Zynga back to its previous heights, it’s more likely that we’ll see both Zynga and its competitors pursuing other ways to grow traffic this year.

Those may include localization for foreign-language markets, new markets like Japan and China, new platforms like the iPhone, and acquisitions of smaller publishers. We’ve actually seen Zynga work toward each of these goals, most recently with a huge investment from Japan’s Softbank and the announcement that it would put FarmVille on the iPhone and iPad.

It’s also probable that Zynga, along with its peers, is constantly learning how to better monetize users, meaning that even declining traffic numbers could still return higher profits to the company. However, without being able to look at the company’s balance sheets, it’s impossible to tell whether it’s preserving last year’s presumably high profit margins.