ok. It works now (for god 's sake). I just check the System Report, FaceTime Built In Camera appears again ( last time it wasn't there). The way to resolve it (i think) is shutting down the system, unplugging the power for around 5 minutes, plugging the power in and then starting the system. I opened Face Time right after the system boot and the camera started to work. Not sure whether problem would happen again or not when I open Skype and FaceTime at the same time. But, yup, there is a clear solution working here at least 😀
Update: Reseting only does not work. I reseted my mac several times already. You need to completely turn off your mac and the power.
Btw, I am kinda satisfied with this mac retina 13 inch. After a long time playing / working with Linux, I finally have a system that can balance between the work and entertainment. Good work, Apple (though I still have Ubuntu in my heart 😀)