• by Gavin O'Malley , Staff Writer @mp_gavin, February 23, 2016

No spring chicken, Instagram’s growth is supposed to be maturing. For now, however, the network seems to be defying the laws of aging. 

This year, 89.4 million U.S. consumers will log on to Instagram at least once a month -- representing 34.1% of all mobile users, eMarketer expects.

More to the point, 51.8% of social network users will use Instagram -- surpassing the 50% mark for the first time -- by 2017.

What’s more, Instagram’s user base is growing far faster than social network usage in general.

In fact, Instagram will grow 15.1% this year, compared to just 3.1% growth for the social network sector as a whole, according to eMarketer.

Over the next four years, it will add 26.9 million users -- nearly double the incremental users expected for Twitter, and far more than any other social platform eMarketer tracks.

To advertisers’ delight, Instagram usage is particularly strong among millennials. This year, there will be 48.2 million millennial Instagram users in the US. And by 2019, nearly two-thirds of all millennial smartphone users will use Instagram.

What’s the secret?

“Instagram and its parent company, Facebook, know that mobile has changed the way people -- especially millennials and Gen X-ers -- communicate, share and get information,” says eMarketer principal analyst Debra Aho Williamson. “That's why they are leading the way in helping marketers reach this audience.”

And, all this growth is growing Facebook’s bottom line. This year alone, 9.5% of Facebook’s worldwide mobile ad revenues -- 20.1% domestically -- will come from Instagram. By next year, Instagram’s share will grow to 14% globally, and an impressive 28% domestically.