You may face many problem in video calling your friend from facebook If you dont have latest version of Microsoft DirectX and also if you are using old webcam drivers .
Recently some user report that they are available but when friend trying to call they are appearing unavailable. For all these type of issue follow below guides.
Here are some things you can do if you’re having trouble with video calling:
Make sure you are using a supported browser.
Make sure another program isn’t using your webcam. Check all instant messaging programs, browsers and video editing software.
Check that your webcam is compatible with your operating system.
If you are using Windows, make sure that you have the latest version of Microsoft DirectX.
Download the most recent drivers for your webcam from the webcam manufacturer’s website.
Some other issues with Facebook video calling include quality of the video, that some users have pointed out is better on Skype. Also there is no way to resize the video window.
Check Facebook Help Center for for more Details