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I've never used the Facebook share function so this is just my guess, but I doubt the device is getting the information about the cover url from in the kepub itself, rather from the metadata that gets downloaded along with the kepub when you buy a book from the Kobo store.
Even if you bought the book from the Kobo store and then sideloaded it as a kepub, I think it would just be treated the same as if you had bought the book from Amazon. (If you sideload a book that you bought from the Kobo store withpout deleting the original, then you will have two books on the device. The two books are treated as completely different books, even though they might be identical in content. Features such as Sync and BeyondTheBook won't work for the sideloaded copy, but will still work for the original.)
The exception might be with some free books that are available from the Kobo store. Some of them seem to be linked directly to the Project Gutenberg site, and if you download the book from the Gutenberg site Kobo treats it the same as if you got it from the Kobo store.
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