How to say "post" as in "post something online"?
Colloquially people use 'PO' (mostly in Southern China), 发, 贴, 放, or any verb that has a similar meaning.
Formal translation would be 发布 or 发表.

Did you see what I posted on Facebook? 你看见我在Facebook上PO的东西了吗？
Where did you post it? On WeChat Moments. 你发在哪了？微信朋友圈。