Twitter co-founder Evan Williams today said he had been told that his microblogging service has been partially blocked in China but had no confirmation."That's what I've been told," he said, adding that he had "heard reports on that" but that the company did not have direct confirmation.Williams said that China can use a firewall to block access to the service."There are ways for users to get around it, but I'm not an expert on that," he said, on the sidelines of the World Economic Forum meeting in Davos.When asked about media reports that Twitter was developing ways to get around the firewall, Williams said he had been "misquoted.""We're not actively developing any way to get around it," he said, although he stressed that "we're for the free exchange of information."Asked if his company had noticed any cyber attacks, such as those launched against search engine giant Google from China, he said that no such moves had been detected.