That's up from the 300 million users it announced in December 2014.
Instgram also says that more than half of its most recent 100 million users live in Europe and Asia, and that more than 75% of total users live outside the US.
"When Instagram launched nearly five years ago, 400 million seemed like a distant dream," the company writes. "Now, we continue to strive to improve Instagram - helping you experience the world through images and connect with others through shared passions."
Facebook has steadily become an app powerhouse. Its Messenger platform now has 700 million monthly active users. WhatsApp has 900 million.
Here's how those numbers have grown since Facebook's last earnings call in March:
As Instagram continues to grow and prove out its advertising opportunities, Facebook's $1 billion acquisition looks better and better.
Compare that to Twitter, which claimed 316 million monthly active users during its Q2 earnings.