From an HT Blog Post:
HathiTrust reached a major milestone on January 5, 2012, exceeding 10 million volumes in its digital collections. More than 2.7 million of these volumes are in the public domain, with viewing and downloading options available online.
The blog post ALSO contains multiple charts including an interactive graph showing growth of the collection over time along with a detailed project timeline (superb!) beginning in January 2008.
Constantly updated collection stats and visualizations are always available here.