If you ask me, this is the starting pistol for Facebook Vs LinkedIn. What on the outside may seem like a new facebook feature that will add to the facebook experience blah blah blah... In my opinion it's the first real formal step that facebook are taking to compete with LinkedIn.
No matter how you dress it up, conference calling is something that first and foremostly appeals to the business community. Yes, I'm sure it'll be great to speak to family and friends, but in actual fact I believe this is Mark Zuckerberg' subtle way of getting work colleagues and your business contacts to 'friend' you on facebook. This, I also believe will make it easier for facebook to launch more business related updates in the future, as your contacts will already be your friends, thus making the transition easier.
So what does the facility allow you to do?
'Group Calling' was initially announced last Wednesday, and subsequently followed-up by updates later that week after questions and clarifications points were raised by the tech community.
It basically boils down to the following:
If for any reason you miss the call, you can join in at anytime and also message someone who needs to join.
For the time being this is audio, however I'm sure facebook will introduce a visual element to this too and allow video conferencing, as and when the technology and/or opportunity allows.