For years, the Center City District (CCD) has been trying to determine the exact number of people that come to work everyday in Center City, Philadelphia. That number has finally been verified; 267,331 private/public sector jobs (or almost 50% of all salaries earned within Philadelphia's borders).
This article shows how important Center City's business district really is to the region as a whole. This study also puts Philadelphia's Center City behind only New York and San Francisco in terms of urban employment percentage in a downtown city district. The CCD is using this data to encourage the city to spend more money on infrastructure and to start decreasing the wage/businesses taxes again, which have both been shown to drive jobs away from Center City.
Feel free to also visit CCD's website to learn more about their mission.