While different than any of FB's previous acquisitions (the service will remain), I think this deal has echoes of the FriendFeed deal. With the FF deal, FB acquired current CTO Bret Taylor who has been, by most accounts, hugely important in helping evolve the product.
As tweeted, innovation of your core use case is amazingly hard at scale. FB cannot ever lose photo sharing. So they are making a bet that the Instagram team can positively disrupt that from the inside.