The Great Firewall of China has helped the government block websites from the Falun Gong to Twitter Inc. for years in the world's most censored Internet market. He Xin recently discovered how to bypass the restrictions.

"I climbed over the wall," said He, an electrical technology major at China's Yanshan University in Qinhuangdao. "Very few of my classmates and friends can climb over the wall, so I can't add them as friends on Facebook yet."

He uses UltraReach Internet Corp.'s free Ultrasurf, one of the growing number of so-called Virtual Private Network services used in the world's biggest Internet market to circumvent censors. Facebook Inc. users in China, blocked in the country since 2009, have more than doubled in the past month to exceed 700,000 after Chief Executive Officer Mark Zuckerberg visited the country, according to data compiled by Socialbakers.com, a site dedicated to analyzing Facebook statistics.

"The number will probably double over next six months," said Jim Tang, a Shanghai-based analyst at Shenyin Wanguo Securities Co., who covers Chinese Internet companies including Tencent Holdings Ltd. "With the visit of their CEO to China, he probably got them more exposure."

Even with the exponential growth, the number of Facebook users in China will be miniscule relative to the nation's 457 million Internet users, the world's largest, Tang said.

Chinese Ban

While VPNs are mainly used by companies to give employees secured access to corporate networks, they can also allow individuals to surf the Internet anonymously because the services employ private "proxy" servers that encrypt data.

"Proxy servers are a key technology that is at the center of how the Internet works, so it can't simply be indiscriminately banned," said Professor Alvin Chan, who teaches computing at the Polytechnic University of Hong Kong. The way the technology is being used to circumvent Web censors is an "unintended offshoot," he said.

For example, a Chinese user seeking to access Facebook would first start an encrypted connection with a VPN service, which would then get on the social-networking website. To Internet-service providers such as China Telecom Corp., that connection would only show the user as having logged on to the VPN server, not Facebook's. The process also works in reverse, enabling VPN users outside of the U.S. to watch geographically restricted services such as Hulu LLC's online videos.