I'm not sure the money is so out of reach for MSFT. FB has 1.3B shares float x $45 = $58B Minus about $10B in cash and it could be had for about $50B. MSFT has about $60B plus room in their unissued shares. The cash flow for both should be good enough for the rating agencies should MSFT take on some debt for this venture.
It would give MSFT a huge boost in social networking. MSFT can't seem to be successful with organic growth into some of these trending IT areas and that is FB's sweet spot. It would also provide some fresh blood into MSFT management (could Zuck eventually take over for Balmer?).
It would certainly disrupt the order of things in mobile, social and search that MSFT can't seem to do on it's own. I also think MSFT will be lost without something significant (if they aren't already there).