Zynga is having a busy Christmas: early this morning, CityVille passed the game developer’s largest hit, FarmVille. CityVille now has 61.7 million monthly active users, and 16.8 million daily actives, both measurements being slightly higher than FarmVille.

CityVille took a total of 22 days to top its 60 million players, a feat that we can unequivocally state is unparalleled in gaming. FarmVille took four months to reach the same point, and it’s the only other social game to ever grow as large.

We’ve written a great deal already about what’s driving CityVille’s growth, including cross-promotion and a large international audience. Changes to Facebook’s platform could also be involved; moving third-party requests back to the notification channel could have helped replicate the viral growth that first made Zynga successful.

The truly impressive detail of CityVille’s growth is that it has showed no signs of slowing — the game is steadily adding over three million MAU per day. With no slackening of growth in sight, it looks increasingly likely that CityVille will reach and pass the 84 million MAU that FarmVille had at its peak, perhaps as soon as the new year.

What’s the peak beyond that? Facebook has a population of well over 200 million game players, and it’s working to optimize the channels that third-party developers use to reach that audience. Sometimes the changes are negative to developers, and sometimes positive; CityVille appears to have been released at a fortuitous moment, which may or may not have been intentional.

With so many available players, it’s hard to say where CityVille will top out, but it’s clear that there is the potential for the game to top 100 million players. Only a half-dozen franchises like Mario and The Sims have ever done so, and no single title ever has.

But CityVille could hit its peak at any point. In the meantime, Zynga isn’t slowing down. This morning, we noticed a Chinese-language version of FarmVille on Facebook, which has picked up over 300,000 players in a matter of days.

Zynga launched a Chinese version of Texas Hold’em in August, but with only language localization. FarmVille has a separate app and an entirely different look, using the extra-large farming plots and bright colors that characterize Chinese farming titles.

The title may be what Zynga’s Beijing acquisition, XPD Media, has been working on since the company picked it up in May. Although the new game uses the same art assets as the original FarmVille, it’s also notably smoother and more attractive.

FarmVille in Chinese won’t set the world on fire like CityVille has — Facebook’s total population of Chinese-speakers is still well under 20 million. But unlike the other language groups on Facebook, most of the Chinese-language users are actually there to game, which makes them particularly valuable to acquire.