The Facetime camera issue happening on Yosemite has boggling quite a long day. Fortunately, there is a workaround for this issue. This happens to be an old issue with older version of Mac OS X, such as Maverick. It is quite simple to workaround this problem.
Step 1: Open “Activity Monitor”
Step 2: Quit “VDCAssistant” process
Step 3: Now it should work !
Here is the source of this fix. Click here to visit