There are only a limited number of things typical people regularly do online, I'd be willing to bet the list for a large chunk is something like
1. Email 2. Facebook 3. Porn
In which case you have a 1 in 3 chance that porn follows Facebook.
1. Email 2. Facebook 3. Porn
In which case you have a 1 in 3 chance that porn follows Facebook.
(email, Facebook, porn)
(email, porn, Facebook)
(porn, email, Facebook)
(Facebook, email, porn)
(Facebook, porn, email)
(porn, Facebook, email)
That is 3! or 6 total possibilities, 2 of which involved porn immediately following Facebook. which is 1 in 3... ...How do you figure 1 in 2?
Edit: On another note, in his place I would have sanitized my own history; a screenshot showing redtube and tube8 in the "visited" style is tacky, to say the least.
Suuuure :)