A foot in Facebook

It's now official [MSFT press release here], Microsoft has taken an equity stake into Facebook:
$240M at a valuation of $15B and few line later "Microsoft will be the exclusive third-party advertising platform partner for Facebook, and will begin to sell advertising for Facebook internationally in addition to theUnited States.".
First rumors of discussions were at $500M for 5% in a valuation around $10B. Then Google entered in the game, then valuation jumped to 12, 13 and then $15B. so why not $20B?
And why the money invested dropped by half?
Because Facebook needs revenue to get its IPO filed next year. An investment would have been below the EBITDA line. Now, how much are we talking about? Facebook might have just secure 3 years of today's advertising with a minimum guaranteed paid of $250M per year. Makes an easy $750M in revenue and $240 in investment, here goes $1B.

Am I far off?