[Update: Facebook has confirmed that the spikes are due to reporting errors, according to AppData’s pages for both FarmVille and CityVille.]

[Update 2: Zynga has also confirmed a technical snafu. “Facebook discovered an insights bug in the reporting of some games as they were testing the new ‘Start Now’ feature. They began working on a fix as soon as the issue came to their attention, and we expect insights to reflect the correct numbers shortly,” a Zynga spokesperson told GamesBeat via e-mail.]

Social game developer Zynga has been in the news a lot recently, and most of it has been of the insider trading and copyright infringement variety. But that streak has paused (at least temporarily) with a report from brokerage firm Sterne Agee analyst Arvind Bhatia, which reveals that the numbers of daily active users (DAU) of two of Zynga’s games, FarmVille and CityVille, saw massive peaks over the past week.

Sterne Agee’s data (see below) show that at the beginning of the month, FarmVille’s number of DAUs rose from about 3.4 million to just under 12 million over a couple of days. CityVille’s numbers went from 3.2 million to 7.8 million over about the same period.

“We suspect it may be related to a special cross promotion within the recently launched game in the Ville franchise, called The Ville,” Bhatia’s report says. After the rise, the DAUs went back down almost as quickly, but they’re still higher than where they started. As of this writing, FarmVille’s numbers were holding steady yesterday at 5.7 million DAUs, and CityVille’s were at 6.2 million, according to metrics site AppData.

FarmVille spike

CityVille spike