Look out, Twitter -- Instagram is gaining on you. Use of the photo- and video-sharing service jumped 35% in the U.S. last year to nearly 35 million people on a monthly basis, according to new data from eMarketer. By the end of this year, the research firm projects that figure will grow another 25% to 40 million.

That would bring Instagram close to the level of U.S. Twitter users at the end of 2013, at 43.2 million. While the two services have clear differences, they share similar demographics and user counts. The microblogging service represented 17.6% of the total Internet user population compared to 16.1% for Instagram.

On smartphones, Instagram has already outpaced Twitter. That’s partly because activity on the photo app takes place almost exclusively on its app. Twitter’s 30.8 million users on smartphones fell short of Instagram’s 34.6 million last year. But eMarketer expects Twitter’s user base on smartphones to grow about 19% to 37.3 million this year.

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