Facebook Inc.-owned Instagram is growing like a weed. The photo-sharing service says it has more than 200 million monthly active users, which is up roughly 100% over the past year.

That means Instagram’s following isn’t that much smaller than the 241 million monthly active users Twitter had at the end of 2013. And Instagram’s user base is growing much faster than Twitter’s. Instagram added 50 million consumers in the past six months, a 33.3% growth rate. Twitter, meanwhile, added 23 million users between the second and fourth quarters of 2013, a 10.6% growth rate.

Instagram says that about 65% of its users come from outside the United States. That means it has some 70 million U.S. monthly active users.

Research firm eMarketer Inc. estimates that 23.9% of U.S. smartphone owners used Instagram at the end of 2013. It expects that percentage to jump to 24.7% this year and 25.1% next year. Instagram already boasts a higher penetration than Twitter, which, as of the end of last year, was used by 21.3% of U.S. smartphone owners, according to eMarketer estimates. The research firm expects that percentage to jump to 22.7% this year and 23.3% next year.

Instagram’s growth has helped it drive its fledgling advertising business that launched last fall. For example, earlier this month it signed an ad buy deal with Omnicom Media Group, the advertising conglomerate that owns firms such as BBDO and DDB Worldwide Communications Group Inc. That arrangement also included an agreement to work with the agency to develop best practices for its advertising program.