Hi there !
I have sold many travel photos and often I find them on Instagram without credits attribution.
I contacted the accounts and they explained that they bought the photos on Istockphoto.
Istock License Agreement says:
Attribution.
Do I need to include a photo credit? You do not need to include a photo credit for commercial use, but if you are using content for editorial purposes, you must include the following credit adjacent to the content or in visual production credits: "iStock.com/Artist's Member Name."
I think the use on social media has editorial purposes. What do you think about it ?