Let's say you really like someone and want to date them but they're with someone else. And let's also say you're a horrible uncaring monster of a human being. Well, now there's an app for that. The Facebook app Waiting Room lets you actively disrupt the relationship of that special someone. It's a little similar to an app called Breakup Notifier, which just told you when certain peoples' relationship status had changed. But Waiting Room goes further.
It also lets you discretely inform someone you're sitting around waiting for them to become single. You can put a spoken-for person in your "waiting room": they'll get an email letting them know, then they can install the app and see everyone out there that's "waiting on them."
Facebook will nuke this in short order as well.