There’s no doubt in my mind that Facebook considers Instagram a worthwhile acquisition. The photo-sharing service has been growing at a rapid pace these past couple of years, even outpacing Facebook itself by adding 100 million people to its list of monthly active users in just nine months. That’s right, Instagram confirmed in a blog post earlier today that, in less than a year, it managed to go from 300 million monthly active users to 400 million, which puts it well ahead of Twitter in terms of both growth and number of active users.