I have not used another’s creative work in the past. However, I have been inspired by artwork plenty of times, and I have not given credit for this. That is because I don’t think that credit has to be given if you are merely inspired by something, and create a whole new piece. If there are visual similarities, than I think it would be in your best interest to credit the artist. I have seen plenty of artists not getting credited on social media, and I know as an artist, this is very frustrating when art you put your heart into is not respected, and you are not given credit.
In the case of the Fairey copyright case, I agree and believe Shepard Fairey should have sued the AP. The “Hope” posters were clearly inspired by his work, and honestly just look like a copy of his work. In this case they could have commissioned Fairey or gave him the rights to the piece.