If your friend doesn't pick up when you call, Facebook lets you leave a "video voicemail" message: You can record a quick video that they'll receive in their inbox the next time they log in.

is supported in Firefox, Chrome, and Safari, but not in Opera. It does not work on Linux.

Add a contact: Adding a Facebook video-chat contact is the same as adding a regular Facebook contact--you have to "friend" the person (and they have to accept) before you can start chatting or video chatting with them.

You should use Facebook video chat if you have the option (it hasn't arrived for all users yet) and if the people you want to talk to are on Facebook.