When I am talking about creating "Facebook only" ads, I am really trying to advocate separate pictures, text, and sometimes targeting through your Facebook Ad manager for Ads that are targeted for Facebook placement.
In my experience, using different images and text for Instagram works better because I believe content is adsorbed in a different context on Instagram. So a distinction should be made setting up an Instagram Ad campaign in Facebook Ad manager.
Instagram Ads are created through Facebook Ads. I believe the Ads you create for these two social networks should be different (Exceptions will apply).