Facebook has a somewhat hidden feature that lets people follow your updates when you're not actually friends with them. Before you freak out, this is only applicable for updates that are shared publicly.
The social network recently changed its friend confirmation functionality (at least for some users). Before, when someone requested to be your friend, you could either confirm them or ignore them. Now, "ignore" has been replaced with "not now". It's unclear if the change is just being tested or is still rolling out. I'm still getting "ignore".
According to MG Siegler at TechCrunch, with the change, users have to go through an extra step to block somebody. He writes, "With the Not Now button, Facebook took what was a one-step rejection and made it at least two steps — and that’s only if you want to truly block somebody (after you click the Not Now button, they ask “Don’t know XXXX XXXX?” and if you click that, it will block them from making any further friend requests). If you just want to deny a person’s request without blocking them, you have to go to the Requests page — the limbo area that Facebook sends the Not Now people to. "
"Facebook has to know that most people are probably going to hit this Not Now button once and forget about it," he notes.
If that's the case, a lot of people are going to be notifying followers of updates without realizing it. Again, it's only with posts shared with "everyone" anyway, but it's still something to be aware of.
It's also worth noting that with or without the change, people have been able to follow your updates the same way if you just let friend requests sit there without clicking confirm or ignore. What's new is that they're making blocking less obvious.