I think he's off here. Rather than 10 users being worth one whopper. I think it should be 10 connections are equal to one whopper. This would make FB worth a lot more than Kottke's $1.8 billion.
Did you read the article? He establishes that on average, FB users have 100 connections, and derives 5 whoppers per user from there.
Oops, he's bad at math, I'm bad at skimming.
That is how he's calculating it. 1 user = 100 connections (on average), divide by 2 because the connections are two-sided, 50 connections, 5 whoppers per user.