A marketing manager does not wake up one morning and just feels it's alright to give 50% on a certain product of his company. Or a business owner does not mindlessly put a specific service as a deal without thinking about the losses and profit.
Before the internet age, making promotions is a marketing science that had to involve pricey surveys and studies. Since most people are on social networking sites nowadays, what most businesses do is let the people decide which product or service is the best one to give a discount for.
On Facebook, some appliance gurus find it effective to share merchandise info and images for them to track how many times subscribers will click on the "Like" button per merchandise. They've transferred the strategy to their respective official websites where their followers can also click on the "Like" button.