4,000 companies have created Facebook directories

Facebook continues to explode on the work networking scene. After launching company directories 3 weeks ago, the number of new company Facebooks created this week has doubled to 2,000, bringing the total number close to 4,000. Those numbers are parabolic, which is a good sign for the health of any viral feature.

Facebook has changed college culture from Palo Alto to Topeka. But how far into corporate middle America can the company go?