In simple terms, if someone has uploaded a picture of a cat to a non-public Freenet key, and someone downloaded it, both you and I have no way to learn about it unless we can spy on those users or flood the network with spying nodes to the point of logging each piece of data, and deducing that these two people exchanged something. Even theoretically, we can only make assumptions about publicly announced data (freesites and message systems), and then try to estimate the proportion of communication that happens in the dark.
And people often make wrong assumptions.