Hi andreaja,
Thanks for the response. I'm the author of the article (Macco, thanks for submitting it!).
You're right that the $50B valuation isn't based on current numbers. It's based on what Facebook might be able to do in the future. Imagine you were buying a house. It's a great house, has a solid foundation, beautiful interiors, and is in an excellent neighbourhood that is constantly appreciating. Would you pay what it might be worth in 3 years, today?
I understand the argument about growth, but even from that end, it doesn't completely stand up. If we were to use the PEG Ratio (http://en.wikipedia.org/wiki/PEG_ratio), for Facebook to be fairly valued at $50B, it has to grow its Net Income by 100% annually, on average over the next five years.
It's possible Facebook might be able to do that next year, but then it'll have 1 billion users. That's 14% of the world's population. That's huge. Being generous though, let's extend that 100% growth into the year after. Then, Facebook would have 2 billion users, 28% of the world's population, and assuming current margins, they would only be eeking out $2 billion in profit.
Will it be worth $200 billion then? I would say no and I would say Facebook's not worth $50 billion now.