Why Instagram’s huge content factories have become a headache for Facebook

On a recent afternoon, a gymnast balanced on his hands atop a third-floor balcony railing overlooking the sidewalk.

A woman recorded the stunt on a smartphone, hoping to capture a this-is-crazy-style video that millions of people will share. That’s the kind of work done all day at 421 Media, a well-funded content firm that produces memes for consumers of Instagram, the social-media platform that in less than a decade has exceeded 1 billion monthly users.

Instagram is part of Facebook Inc. ’s social-media empire, and its continued growth is essential to the parent company. Yet the platform has grown so big, so fast and so commercial that it risks losing the stylish and intimate aesthetic that made it wildly popular.