Less than three weeks have passed since the last time we wrote about Applifier, which offers Facebook developers a cross-promotional bar that is shown to players alongside games. But in that short time the company has already added 20 million new monthly active users across the network, according to CEO Jussi Laakkonen.

Applifier is becoming more public today, opening officially to any Facebook game developer, with a new web page showing off top clients and how the bar works.

But the real news, as it was in August, is the size that Applifier has grown to. As a network, it’s now larger than any Facebook developer with the exception of Zynga — Electronic Arts comes a close second with 54 million.

The promotional network has been growing so quickly because there are a large number of developers, like ZipZapPlay and Booyah, who have millions of monthly active users but only one successful game, which means that they can’t do significant cross-promotion on their own.

Mid-sized developers like Wooga and Digital Chocolate, who do have multiple games, are also taking part. Laakkonen says that Applifier has had to concentrate almost entirely on scaling since its launch four months ago; it now delivers 200 million impressions daily.

Asked whether a top five developer like Playdom could potentially take part in Applifier, Laakkonen says it likely won’t ever happen. “When we think about ourselves, we think about leveling the playing field,” he says. “The whole point is the portfolio. Zynga, Playfish and so forth can build that portfolio for themselves, and they can try to capture the whole game-playing of a particular player.”

At the same time, Laakkonen is still coming to grips with Applifier’s growth to date. “When we started four months ago, if someone told me we’d cross 50 million, I would have laughed. At 10 million, I thought we’d made it, that there was nothing more to be done,” he says.

Although Applifier does paid campaigns, its main premise is that player traffic between games is free; it’s now up to 10 million click-throughs, according to Laakkonen. For more on how the network works, check out our stories from its launch and 35 million milestone.