Facebook works in collaboration with Skype (popular software used for video calling). In order to set up video calling using Facebook then you and the person who you are chatting with, should a compatible webcam. Once you and your chatting friend meet this requirement, then the video button on Facebook will automatically become available and then you can initiate a video call with chat support as well. The quality of the video strongly depends on the speed of your Internet connections and such you may hear delays or lag between sound and image.
