Cosmologists have measured the real universe with greater precision than any reportable metric encompassing the extent of the digital universe — even though much of it is sitting on our desks.
Sales figures of all kinds are readily available, whereas absolute numbers (of processors, CPU cycles, addressable memory, disk space, total lines of code) are scarce. We are left to make rough approximations (skewed by volatility in prices) where there could and should be a precise, ongoing count.
Have we reached Avogadro's number yet?