• by Mark Walsh  @markfwal, March 27, 2014

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.

With its step Wednesday to add photo-tagging and the ability to share multiple images per tweet to its iOS and Android apps, Twitter is taking more direct aim at Instagram, as well as parent company Facebook. It’s not the first time Twitter has taken a page from Facebook, and probably won’t be the last incidence of the two rivals adopting each other’s features.

Given that Twitter recently passed its eighth year, eMarketer reports that the make-up of its user base is maturing, mirroring the general population more closely. People aged 18 to 44 are estimated to account for 60% of Twitter users this year. By contrast, Instagram is still largely confined to Millennial and Gen X users, with 70% falling in that age range.

“Over time, Instagram's user base in these age groups will approach but not surpass Twitter's, and eMarketer does not expect significant shifts in usage by age for either site within our forecast period. Instagram's user count among users aged 18 to 44 will remain about 1 million fewer than Twitter's in each year throughout our forecast,” stated the eMarketer report.

One audience change that is well underway for Instagram is a shift toward more gender balance. In 2012, about two-thirds of its U.S. users were female. While women still make up the majority of users, the ratio is expected to reach 55% female vs. 45% male by 2016.