The biggest threat to Facebook in China, however, is unlikely to be either of the leading Facebook clones, RenRen or Kaixin001, said P.T. Black, Thoughtful China’s senior creative director in Shanghai. “I declare 2011 to be the year of Weibo,” namely the leading player, Sina Weibo, which has around 140 million users.

“We’ve definitely seen [Sina] Weibo taking time away from time spent on RenRen,” said Eugene Chew, a digital marketing consultant, plus Chinese consumers use social media sites differently from western consumers. For net users under age 35, for example, “the internet is a primary medium not just for self-expression but for entertainment. They play games more than sending e-mails and they’re watching a lot of movies and TV shows online.”

Facebook faces yet another obstacle, the need for self-censorship, Mr. Harrington noted. The challenges Google faced would be “amplified” with a company like Facebook, which “has no experience policing or monitoring what’s being published.”