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Facebook’s IPO filing gave us an inkling of how much money the social network expects to make when it floats this year, but also a hint of what it could spend it on: the first true Facebook Phone. In among the lengthy IPO documentation were not only details of Facebook’s existing achievements in mobile but a map of the challenges yet to come along with Mark Zuckerberg’s biggest fears for the future. $100m is enough to stage a big attack on the mobile market, though, and there’s one obvious place to start: HP’swebOS.