Since its debut in 2010, Instagram has accrued an impressive 300 million daily users. But with the ascendancy of 4-year-old Snapchat, which now boasts 150 million fans and counting, Instagram is looking over its shoulder. Their fierce rivalry for the hearts and minds of millennials and Gen Z touched off in late summer when Facebook-owned Instagram added disappearing stories to the mix, clearly mimicking a feature that’s been key to Snapchat’s success since day one.