Pizza lovers have to pick their poison to rack up free pizzas. Pizza Hut, Papa John's, Domino's (and others, I'm sure). All now have 'earn a freeby' programs that are pretty clear and friendly. Hut Rewards has generated a lot of excitement because during launch--now until October 1, you can earn a free pizza by spending as little as $50 online.
The important point is that you've got to stick with a supplier! Online and mobile ordering are also usually required, and you'll still pay tax, tips and delivery charges.
I expect this to become the industry standard, like frequent flier programs at the airlines. Which brings me to an important point. Many people gravitate to the most generous program, but some of us are more concerned about the experience. Pizza is not a big enough part of my budget to justify anything but my favorite slice.
So watch these programs develop over time, and let's see if Pizza Hut continues to be the leader or if they jockey for position.