Move over, Twitter. Photo-sharing service Instagram is now approaching the user levels of the older micro-blogging service.

Almost 35 million Americans used Instagram at least once monthly in 2013, according to research firm eMarketer, compared to 43.2 million for Twitter.

Instagram is heavily tilted to younger users, while Twitter’s user base has begun to even out among age groups. And, although more males are coming aboard, Instagrammers were 59 percent female last year. Twitter was nearly even.

A younger, more female and more socially active user base may be finding Instagram a more hospitable service.

“[You use Twitter] more to form relationships with media entities like TV shows” or to follow celebrities, Reticle Research principal analyst Ross Rubin told VentureBeat.

“Instagram is a way for circles of friends to exchange photos.”

Rubin noted that, while Twitter has added media capability, it still remains a kind of micro-broadcast service. There’s a reason, he said, why the more sociable Instagram is a “good fit” for its parent, Facebook.

Earlier this week, Instagram said it had passed 200 million active monthly users worldwide, with 35 percent of those in the U.S.

When asked about the different figures — 35 million monthlies in the U.S. versus 70 million reported by Instagram — eMarketer told VentureBeat that their methodology differs from the photo service’s and pulls data from a variety of sources. But the higher Instagram figure could also reflect huge growth rates in the months since eMarketer’s averages from last year.

Twitter, started in 2006, is four years older than Instagram – a period during which mobile boomed and became the device of choice among younger users.

“Even though Twitter was built with mobile in mind [for its 140-character SMS messages],” Rubin said, “Instagram has embodied not only ‘mobile first,’ but ‘mobile only.'” Instagram, he noted, is built from the idea “that a camera is integrated into your smartphone.”

The photo-sharing service also has “a simpler learning curve than Twitter,” Rubin said, “since it was invented for the iPhone and was [originally] exclusive to it.” He also pointed to Twitter’s history of multiple clients, usage on non-mobile devices, and an “inconsistent” rollout of features across clients and platforms as potential slowdowns to user acquisition.