Well it's actually in the rules of that FB group to mention the original artist if it's fanart, so it's not like a link to the source is nowhere to be found on the repost.

facebook(dot)com/groups/fkg.en/permalink/1722260518021302/
6) Always link the source for any fan work you post. This should be the artist's pixiv page or twitter post or similar, not aggregators like booru sites.
Apologies if I sounded antagonistic or what. I do mean no ill will here, and I'm sure you just want to share some art for the sake of it. It's just that it wouldn't hurt crediting the artist if you ever repost one. Who knows, there may be art enthusiasts around that like to take a look at more from the said artist. Peace.