How are dependencies such as external libraries counted? Some programming languages will "include" code while others will link "against" external code.

Not sure how they measure the lines of code. Are comments, headers, etc included or is it just the the code flow?

Does an average modern high-end car really have a hundred million lines of code?

The number of LOC is nothing to boast about in my opinion. The simpler your codebase the easier it is to maintain, the easier it is to focus on what actually matters: the end product.

It also seems quite arbitrary to compare LOC's between all those products. It seems like it would vary greatly depending on the language and other factors (No idea of the metric includes comments/whitespace etc).

That last one is a hoot.

Does 60 millions of lines of code of Facebook make sense? I mean I know its a large project with multiple modules, but any ideas why such an enormous codebase?

The original Doom was under 50,000 lines of code.

Firefox and Mozilla core are games?