Many people have been able to do this for a long time (sort of) IF they had a VOIP (voice over internet protocol) client installed, such as Skype or Vonage, or, if they are using Facebook mobile on a smart phone. Facebook, however, has now made it possible to call you friends using the video-chat feature, including group chats (think "conference call").
Of course, your friends will have to be on Facebook, and tech-savvy enough to use the feature, so Facebook isn't going to be replacing other Vonage or Google Talk (which allow users to call any phone). But it's a great concept, and it will be interesting to see if the performance is any better than Facebook chat has been in the past (when dropped connections were frequent). If you would like to try it, click HERE, or it will be automatically available on your account eventually.
Speaking of Google, I'm sure it's purely a coincidence that Facebook is rolling out this feature just as Google is rolling out its Google Plus, which is in invitation-only beta testing, but aims to be Facebooks one and only competitor.
Of course no announcement about video calling would be complete without a video of smiling happy people saying hello to each other in different languages. Get your tissues out, and enjoy.