"I came up with the idea that, why don't we, if it comes to it, say a number that's so outrageous that no one could ever possibly accept," Stone recalled in an interview with HuffPost Live's Caroline Modarressy Tehrani.
"I thought of the biggest number I could think of -- $500 million -- and we laughed until we couldn't breathe about how ridiculous it sounded."
What Stone and Williams couldn't have expected was Mark Zuckerberg's willingness to pony up.
"Mark said, 'that's a big number,'" Stone recounted. But sure enough, "...the offer came through. [Facebook] was like, 'okay, we'll do it.'"
Ultimately, no figure -- not even $500 million dollars -- could motivate Stone and Williams to sell the platform they'd nurtured since its conception.
Williams went on to write "a really respectful note" declining the offer and explaining that the two wanted to build the company and see it through themselves.
To find out more about Biz Stone's experiences with Facebook, pick up his recently released book "Things A Little Bird Told Me."
Watch Biz Stone's full interview with HuffPost Live below: