Both post and posting are the same according to Cambridge Dictionary (Android version). Both have the same meaning i.e.
an electronic message that you send to a website in order to allow many people to see it
Anyway, these are the screenshots that I took:
Related to the question, what's the difference? To give a particular example, suppose, I want to refer something that uploaded by someone on social media, is it a post or a posting?
P.S. Please use a vpn if the images can't be seen. It's probably your country blocked the imgur feature. That works on me. It's difficult to tell what's on the screnshoot when the definition of both two are exactly the same.