Ooops – I’d spent a lot of time looking around wordpress for an answer, then just realized I hadn’t tried to find the plugin’s faq. I’m using the addtoany plugin and just found instructions for finding the data in google analytics. Until then, I thought google would only show subscribers who use the google reader.
For anyone else with this question, it’s simple:
In google analytics, when in “Top Content”, type in /addtoany.com/subscriptions in the find page function at the bottom.