Seasonal Facebook Defection Disorder?

Facebook shed some 400,000 members between December and January in the United Kingdom. This according to new figures from Nielsen Online, which charted a 5% decline in U.K. traffic month-to-month.

Which begs the question: Is Facebook nearing its saturation point? Is enthusiasm for the social-networking phenom finally wearing off? Have we all been spammed by the ironically named “Funwall” one time too many? Are the site’s privacy issues finally taking their toll? Or are its zombified members too busy seeking human flesh to bother updating their profiles?

Or were they simply on winter holiday?

That last scenario seems the most obvious explanation. December and January are the months at issue here. And Nielsen’s figures show that there are 712% more Facebook users than a year ago. Still, this is the first drop the firm has recorded in Facebook’s user numbers in the U.K. since the site became large enough to track. There wasn’t a similar drop in usage last year. Or the year prior. So maybe there is something more here. The early beginnings of a long-term erosion, perhaps?

“One month of falling audiences doesn’t spell the decline of Facebook or social networking,” said Nielsen’s Alex Burmaster. “However, most of the leading social networks are less popular in the U.K. than they were a year ago. It was inevitable that early growth rates couldn’t be sustained and the larger networks have been plateauing over the last few months.”

Seems the leading social networks to which Burmaster refers were also less popular in the U.S. According to the latest stats from comScore, Facebook attracted 33.9 million unique visitors stateside in January–down 2% percent from 34.7 million in December. That’s a decline of approximately 800,000 users. Again, this drop could also be chalked up to Seasonal Facebook Defection Disorder. Or not. After all, it’s not like we haven’t seen this sort of thing before. Remember Friendster?

UPDATE: Facebook disputes Nielsen’s metrics. “The number of users for Facebook continues to climb in the U.K.,” the company said. “Our internal monthly active user numbers rose between December and January in the U.K. and are now at more than 8.3 million. Facebook tracks active monthly users, rather than registered users or unique visitors. Active users reflect those who have used the site in the past 30 days.”