NEW YORK (AP) — It's a record for Facebook. For the first time, a billion people used Facebook in a single day. It happened Monday — and CEO Mark Zuckerberg marked the occasion with a post on his Facebook page. He notes that one of every seven people on Earth logged in to the social network on Monday. Overall, Facebook has nearly 1.5 billion users who log in at least once a month. It hit the 1 billion user milestone in October 2012.