Ellen Dubinsky has published "A Current Snapshot of Institutional Repositories: Growth Rate, Disciplinary Content and Faculty Contributions" in the Journal of Librarianship and Scholarly Communication.
Here's an excerpt:
Mean and median growth rates of IRs have increased since measured in 2007, with variance depending upon size and type of academic institution and age of the IR. Disciplinary content in IRs is unevenly distributed, with the Sciences predominantly represented. IR administrators remain actively involved in the submission process and in the promotion of their IRs. Personal contact with individuals or groups of faculty is the most used and successful interaction method.