Etiquette for publicizing postdoc position
You are unlikely to offend a reasonable person by revealing you got a job.  So I would say there is not an etiquette beyond avoiding excessive boasting.  Don't talk about how you are better than the people who did not get the job.  Don't talk about how your job is better than other people's jobs.  You probably knew that.
This is not etiquette, but:  If you get a job &quot;early&quot; it is in your interest to keep it secret for a while.  A lot of people think that &quot;I have a new job starting in September&quot; is the same as &quot;I am leaving tomorrow.&quot;  They will quit contacting you and quit collaborating with you because they are confused.  You will constantly have to answer questions about why you are still around.