Facebook currently has very limited editing for their pictures. By acquiring a well-known and talented company, Facebook can roll this into their current offerings. I would be very skeptical that this was a user-driven move. I can't imagine a large percentage of Instagram users who are not also Facebook users, even if people posted a lot of Instagrams to Twitter.
its not about new users. it is about engaging their existing users on a new platform - mobile. Pretty sure instagram mobile crushes facebook mobile in engagement
one, instagram has brilliant team which also include creative people and not only tech.
two, very high yet active user base.
three, which is purely on business value - the cumulative time spent in the mobile app of 30 million active instagram users, when you add up, will give more revenue in facebook ad in the following years. i would say, facebook played safe here in trying to regain the usage time.