When Facebook reported its second-quarter results in July, the company said it had an average of 968 million users logging in daily.
But Monday of this week marked the first time a billion users logged in on a single day, CEO Mark Zuckerberg said in a Facebook post on Thursday.
Looked at another way, one in seven people on Earth logged in, he wrote.
Zuckerberg did not break out the statistic further, or say what might have driven the high number of log-ins on Monday.
Over the past few years, the number of the company's users on mobile has grown more strongly than its desktop users. Usage on mobile devices surely factored into the new milestone.
Facebook's Internet.org project may also have helped. That initiative provides access to certain Internet services -- including Facebook -- in under-served markets. It's currently available in more than a dozen countries, including India, Kenya and Colombia.