Mac: For people who use Facebook more as a communication tool, it's often not worth the potential productivity drop to head over to the main site all the time. Glow is a Mac app that aggregates your Facebook notifications into Notification Center without being intrusive.
The nice thing about Glow is that it's easy to customise to fit your needs. You can even set it up so that it doesn't show alerts, but instead just lights up and tosses those notifications into Notification Center. It makes it so that you can still access notifications easily, but it doesn't pull you away from whatever you're doing. If you're not on Mountain Lion, Glow works without Notification Center support.