People have been asking me how I got the 500 million figure.First a few numbers. Count of Mobile numbers: 5 billion , count of Mobile FB users:600 million.
I get at least one hit for ever 10 numbers.The probability of a single number being set as "everyone" is 0.1 .Therefore 0.1 * 5000000000 = 500000000.
I agree I wrong about the 98% of Facebook user being vulnerable part . It is actually 83.3% .