David Bigwood on his blog stats:
Making your content available in more places makes metrics hard. Before Bloglines, Google Reader, Facebook Blog Network, Planet Catalog, and all the rest I could get a feel for the number of readers. Didn't matter too much to me, this is done for my own benefit as well as the community. However, if I was in a position and needed numbers to justify the work it would make it difficult. [Catalogablog: Facebook]
David's comments were in the context of the Facebook Blog Network.
I agree. I also don't have a good sense of what the relative balance is between reading 'elsewhere' (in Bloglines for example) and reading at the blog site itself. Nor do I have a good sense of how many people use 'personal' readers (I use the reader built into Flock, for example) as opposed to network level ones (Bloglines). Some stuff I could find out with a bit more effort, other stuff would be much more difficult.
It does strike me that this is an increasingly common pattern. We 'disclose' materials in particular ways, and then they enter a diffuse network of syndication and synchronization.
On a related topic, I look forward to seeing some numbers from those libraries who have been experimenting with Facebook apps, toolbars, etc, to see how much use they are channeling to the library.