Facetime suddenly inaccessible on my MacPro.

I had FaceTime for eight years, but it recently disappeared from my dock and my MacPro. The app store says it's only available for iPhone and iPad. Has it been taken away by the Apple Gods? Is there a way I can reinstall it? Why do things come and go on the dock at random, and why can't I delete things I don't want and have them stay gone? Is it because I allow updates?