Facebook probably would have copied them somehow, though I don't know if it would have been successful. FB's duplicate of Snapchat has been a huge success and is a major part of IG today. I don't know if FB would have been able to successfully replace Snapchat without using the Instagram app as a springboard.
Instagram Stories seem like a logical extension of Instagram itself. Adding a pure photosharing section to Facebook or something like that in order to copy Instagram seems much more redundant and difficult.
To be fair though, the minds of Facebook have much more incentive to find a good way to do so than I do
they did copy them, it failed. it was called "facebook camera"
edit: they also had a snapchat clone called, "poke" that too failed. the only reason they got to snapchat was because instagram was so popular and they just went ahead and copied the most popular aspect of snapchat: stories.
