Honestly, though, doesn't using VoIP seem way harder than pressing one button on Facebook?
As someone who's never used it, I would have to go through the effort of Googling for a client or an online app, figuring out how to use it, adding my billing information and then actually paying for it, when Facebook is completely free!
Also, this is always a good time to bring up how ridiculous international calls are from the UK for anyone who doesn't know. [1] My 22 minute Facebook conversation with my nan over Facebook last night would have cost me £22 + a small connection charge.
[1] http://www.vodafone.co.uk/discover-vodafone/managing-my-cost...