http://www.census.gov/ces/dataproducts/datasets/lbd.html
http://www.census.gov/ces/dataproducts/bds/overview.html
Methodology:
http://www.census.gov/ces/pdf/CES-WP-02-17.pdf
Looking more carefully, I think the numbers tell a completely different story - this is not about the decline in entrepreneurship, but more about the rise of contingent workers and one-person businesses. Their definition of firm is a business that has paid employees and not all contingent workers count under that definition because they are often independent contractors or employees of a third party agency. The broader availability of all kinds of services from legal to insurance to accounting to technology to logistics to HR/staffing also allows 1-person businesses to handle more than they ever have, while potential legal and regulatory concerns have raised the cost of hiring the first employee. This is not about consolidation but bifurcation.