I thought I read before that Facebook tried to buy Instagram last summer but it didn't happen (my guess is that they offered $500m). I think FB might have offered too low last summer, and has been watching Instagram ever since. Instagram's growth is amazing and they have the network effect in the mobile photo market. I personally think Facebook got a great deal at $1b and that Instagram will continue to grow and reach 100+ million users within the next couple years. $1b to acquire Instagram will look cheap in a few years.