This is true.
"therefore, yes, they are valuing the company at $80bil or whatever the number is."
But this does not follow. They are valuing their investment at the price they paid. They aren't (necessarily) valuing the rest of the company at all.
For instance, they could believe that Facebook's total value will crash to nothing in the next couple of years, but that (due to demand for Facebook stock) they will be able to make a 500% profit by selling off their investment beforehand.
That's an extreme example, but illustrates a problem with extrapolating a company's value from the sale of a small percentage.
I'm sure others have studied this, but intuitively something like a rolling average seems to make more sense for that sort of extrapolation.
Of course, GSV is simply betting that Facebook will be valued by the public markets for more than $80bil at some point in the future, not necessarily for any extended period. They might very well be hoping to make money off Facebook in a greater-fool-theory play.