A large part of Facebook's appeal is the ability to add apps to your account. Games (such as Farmville or Vampire Wars), social apps (like Yelp or horoscope apps), self improvement apps (dieting apps, investing tips) and many more can be found on the site, many of which are free.
There are times where Facebook will determine that certain apps violate their terms. For example, in early 2009, Burger King created an app that, if you dumped 10 friends, would give you a free Whopper. Really. Facebook's rationale was that apps are not allowed to tell people if you've dumped them as friends, and this app did.
Facebook just pulled the plug on an app that many contended was a stalking app. Breakup Notifier was an app released recently. The whole purpose of the app was to let you know if someone's Facebook relationship status changed. For example, if someone was listed "in a relationship" and changed their status to "single", you would be alerted.
Many people view this as a stalking app, but in my opinion, if someone puts this information out there publicly, there really isn't anything wrong with it. I could just as easily bookmark a profile and revisit it to see if someone changed their profile status, and I would also be able to see it through my News Feed. This app would just monitor it for you and send you a response via e-mail when that status changed, providing you with real time information.
Facebook has apparently permanently disabled the app, citing some reason or another. Was it the 3.6 million users it amassed in a week (and the strain it added to their servers), was it complaints, or was it something else? I would guess complaints. I am assuming that the same people who post their relationship status publicly are the same people who wrote to Facebook to complain.
Undeterred, the creator of the Breakup Notifier has created another app, Crush Notifier. This works very much like speed dating. Let's say Sally has a crush on Tim. She would mark that she has a crush on him, using this app. Tim would not receive notification of Sally's crush, so Sally does not risk rejection or awkwardness. However, if Tim marks through the app he has a crush on Sally, they would both receive e-mail notifications that they like each other. Crush Notifier even has a business model, where you get two uses for free, and would need to use Facebook credits to receive any further e-mails.
Amazing how social media continues to change society, huh? Well, as Ricky Bobby said, "Does that blow your mind? That just happened!"