Back when I did farmville, they had events with companies. If you liked the companies page, you'd then get that special item unlocked in farmville. Forget to unlike the company, and you tend to get their spam messages.
Ever since I've unliked farmville and other games, I still get their spam on my page. I tend to ignore it as I don't go to facebook as much as I used to. Again, it's spam I, as the user agreed to.
There's those of us who signed up for Gmail years ago. This wasn't in the original TOS. Under duress to keep an account, it's spam. Google had to discontinue Buzz.