Recently, there are some Facebook users who have spotted a notification on their Facebook feed saying that they are able to view Instagram Stories directly on Facebook. The stories will be found within the Facebook Stories feed and will be surrounded by an Instagram rainbow border around the profile picture.
Since this was first since, it was since confirmed this news:
In order to see an Instagram Story via Facebook, you have to connect the user on both Facebook ad Instagram, as well as having that user link both their Instagram and Facebook accounts.
Source – Social Media Today