A lot of people seem to be arguing that this move may have been not to get Instagram tighter with Facebook, but rather to prevent any other platform or company from acquiring Instagram. Ergo, it's a move by Facebook to further cement their lead as 'the place you share stuff'.
Ok, if that's true, in what way does buying Instagram 'further cement their lead'? Isn't Instagram really, extremely easy to code on your own? I'm in the process of learning Rails and I feel confident I could bang out a Heroku app using S3 for storage to upload/display/process photos. Then all you need is a thin native device to grab photos from the library or from the camera directly and upload it to that app. You don't need to have any platform-specific code outside of handling the camera and photo roll on the device.
Point is - did this do anything to stop, say, Google Plus from making their own Instagram?
Another related argument is that this means that the existing user-base of Instagram will get folded in to Facebook. Personally, I would be flabbergasted to find that more than 0.1% of Instagram users don't already have Facebook accounts. And if you are not using Facebook for photos already, you're either not going to want to share them at all, or else you've already got someplace else you want to share those photos.
Basically - where is the value in this acquisition? I mean any value at all? It seems like they paid 1B to avoid having to develop a very very small app.