Let's imagine we have three blogs: A, B and C. A has 10, 000 readers, B has 100 readers and C has 10 reader. Let's also characterize the topics that these blogs write about. A writes about topics t1, t2, ..., t10, B writes about t5 and t6 and C writes about t5 only.
Now, suppose C writes something interesting on topic t5 and both A and B links to this post adding their own particular commentary. Who will drive more traffic to C? A or B? While A has many more readers than B, it is topically a very broad blog. The writer doesn't have the time (or expertise) to really go deep into the issues of all these topics. Consequently, her audience is not made up of experts in those areas and reads there to get a high level picture over a broad range of topics. On the other hand, B's readers pretty much just go there for 2 topics. B has time to go in to detail and understand those topics and is probably spending plenty of off line time in that topical space as well. Consequently, B will actually send more traffic (not relatively more, but absolutely more) to C than A will due to the specialization of his audience.
What I'm describing above is the difference between some notion of popularity (which may be called influence) and some other notion of authority (or expertise) and how these issues are related to both the blogger (blog) and the readers of that blog or feed. Measuring readership on topics is key to really modeling this stuff in social media which is why FeedBurner is such an asset to Google. It also captures why metrics for bloggers should capture notions of topic (something which BuzzLogic understands).
[Thanks to Akshay Java for discussions that highlighted this issue.]