Skype added 39 million users in Q4 2009, to come to a total of 560 million accounts.#2
Its still early for facetime, but I wouldn't discount Apple, especially with over 120 million iOS devices sold.#3
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#1 http://www.zdnet.com/blog/btl/business-week-14-million-googl...
#2 http://gigaom.com/2010/04/20/skype-q4-2009-number
#3 http://mashable.com/2010/09/01/apple-ios-downloads-stat/