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

The draw of 1 billion monthly users has spawned an aggressive behind-the-scenes advertising economy that threatens the site's originality and appeal

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