There are 8 message slots and the order is determined by "points". The number of points initially equal the amount of clams sent to my address. The points decay at a rate of exponentially with a half life of 60 days.
More details are here: https://mathgate.info/clamspeech.php
For people who don't know about clams, the main thread is here. Clams were distributed to bitcoin, litecoin and dogecoin addresses with nondust amounts in May 2014. If you have one of these addresses and haven't yet "dug" your clams, the clams at your address are currently worth about $16.50.
Edit (Sep 10 2015): There's low enough demand for this that most of the ad slots were reaching negative points. I decided to change the way points decay from linear to exponential so that negative points will be impossible. The post above has been edited to reflect this change.