I think this rumor has already been debunked yesterday...
But for the joke's sake: remember that Microsoft payed 5 billion $$ for "Minecraft" and then only offering 16 billion $$ for Valve. That's beyond ridiculous.
Valve is not a public company. It's a private company, and their administration isn't driven by public investors. They can't be bought unless they're willing to being bough. And I can't see any future where Valve would want this.
A rumor is currently doing the rounds that Microsoft wants to make a purchase offer of 16 billion US dollars to Steam operator Valve. However, there are good reasons to doubt the veracity of this statement.