I went to career.com to see what they call their ____s. They call them listings.
You could also use a verb: "I saw your job listed at career.com, and ..."
In newspapers these are advertisements.
I think that "posting" is normally used within a company. They post new job openings so that current employees can apply for more advanced jobs within the company. Formerly this was done on a bulletin board (the cork kind); now surely it's on the company's internal web site.