Absolutely not. You couldn't be more wrong. The servers went from nearly zero to four million new devices hammering it hard while everyone exhausts the new feature over and over and over again showing friends, family, etc and learning it themselves.
As the initial huge chunk of new 4S devices tapers off, new 4Ses are activated at a steady, lower rate, and people will get used to Siri and not use it as much. Also, I wouldn't doubt the servers are going to be getting periodic upgrades.
It's common sense for a new feature to be used way more during the period when it's new, and then taper off to normal use later on.