Instagram has announced that it has reached the milestone of 500 million monthly users. That's a commonly used metric that has its limitations when it comes to tracking just how engaged users are with a given app, but Instagram says that over 300 million of those people use the service every single day.

There are now over 100 million monthly Instagram users in the US, and the global userbase has doubled over the past two years; Instagram reached 400 million monthly users in September after hitting 300 million the previous December. The company says a typical day sees over 95 million photos and videos shared, with 4.2 billion likes on average.

While Instagram's continued growth is encouraging considering the struggles of rivals like Twitter, there's still quite a bit of headroom for the service. Fellow Facebook acquisition WhatsApp has over a billion registered users, while Facebook itself serves over a billion daily users.