Literally hundreds of millions of people. McDonalds alone has 62 million followers on Facebook. If half of the people on Facebook or Twitter follow a brand, that's over a half billion.
I have to assume at least a good chunk of those followers did so in order to enter some promotional, giveaway event. But even if we assume half did so to win something and account for bots, that's still millions of people who voluntarily decided to follow a fast-food chain. I'm with the grandparent, I just don't get it.