By Robert Cyran and Rob Cox
Facebook may at last become a real, rather than virtual, money spinner in 2011. The social network doesn't need capital to grow, but expectations from backers and employees for an IPO are too high for founder Mark Zuckerberg to ignore. There's just one challenge: trading in Facebook's unlisted shares values the website at more than $40 billion. Getting to that number requires some astonishing growth assumptions.