A.) Enter an existing revenue space occupied by entrenched players (likely Google, Yahoo, Microsoft, Linkedin, Amazon, Ebay, Apple) and grab a big chunk of market share large enough to validate the $50-70 Billion valuation. With some of the competitors having cash warchest almost the half size of Facebook itself.
B.) Create a brand new way/market to monetize. Has to be around $6-15 Billion/year revenue to justify valuation. Quickly, before they run out of money/hype. In a maturing/saturating online space. (for comparison, Digital Music is a $15 billion dollar market)
On top of that, they have to be mindful of the declining game ad spend, declining monthly active game players, and the increasing costs from users overseas that cannot be monetized.
I am sorry, I just don't see it.