A while back Coles opened up a Twitter account (@Coles) which got flamed from the start.
They immediately abandoned it and it was desolate for some time, but it now seems that they are responding to their tweets trying to fix their "customer service".
Using twitter to complain is a great way to vent your anger publicly, everyone can troll though the previous tweets and see what a others are complaining about.
Apart from twitter, there are other ways to complain, all of which are good to follow up on your public complaint with private details, such as name and address.
1800 455 400
@auspost is another well supported twitter page, where hundreds can complain and the rest of us can look down on Australia Post's performance.