The simple way to think about Instagram is that it's a play on nostalgia--that it looks to recreate the sense of a previous era's photographic aesthetic for the purposes only of making pictures seem vintage.
The problem with this view is not only that it ignores an important reason why people actually seem to use the app. That is, it seems to foreclose a discussion of the emotions that we are trying to evoke with the app's various filters, focus options, captions, etc. Rather, it also seems to paper over the question of how the form of the app is changing what we think constitutes photography.
How does Instagram change our understanding of the snapshot? What are users trying to tap into other than nostalgia?
A framing thought: the photo filters *don't* really aim to recapture any lost sense of how photos creates meaning or induced emotion. When these filters weren't filters (that is, when it was just the way analog technology worked), the style would have seemed eminently modern. So at the very least Instagram actually warps the sensations created by the filters in the very act of trying to reproduce them.
I dunno. I'm on a bus. Thoughts instagrammers?