A day after Secretary of State Hillary Clinton's pledge to promote Internet freedom, efforts by U.S. diplomats to generate debate on the issue on Twitter-like microblogs in China—which has the world's most Internet users—ran up against the country's sophisticated censorship system.

The virtual tussle on Wednesday highlighted the growing importance of such microblogs in China as a new frontier for lively discussion and information sharing that is so fast-paced that censors often have difficulty keeping up. It also pointed out the growing efforts of the U.S. Embassy in Beijing in trying to interact with Chinese citizens.