Excerpt from this article:
Many people with public accounts on Instagram may not realize that when random users follow them or like or comment on their posts, it is often the work of a cottage industry of websites that, for as little as $10 a month, send their clients’ accounts on automated liking, following and commenting sprees. It’s a rogue marketing tactic meant to catch the attention of other Instagram users in hopes that they will follow or like the automated accounts in return.
The fake activity has long violated Instagram’s rules but has proliferated nonetheless. In the last seven weeks, however, Instagram, the Facebook-owned app with 700 million users per month, has been cracking down on such businesses. It’s led to the shuttering of anonymously run sites with names like Instagress, PeerBoost, InstaPlus, Mass Planner and Fan Harvest, though others remain.