SAN FRANCISCO: One in seven people on planet Earth used Facebook in a day, CEO Mark Zuckerberg told his followers on the social network Thursday. “For the first time ever, one billion people used Facebook in a single day,” he said in his posting. On Monday, “this was the first time we reached this milestone, and it’s just the beginning of connecting the whole world.” The human population is 7.26 billion, according to the U.S. Census Bureau. In July, when Facebook released its second-quarter financial results, the company said it had an average 968 million daily users. About 1.5 billion users checked their account at least once a month.