ContactBabel has publshed the 4th edition of the their US Contact Centers 2016-2020
The State of the Industry & Technology Penetration.  See: http://contactbabel.com/pdfs/oct%2016/US-SOITP-2016-Marketing-v2.pdf

Some key findings:

• There are 41,500 contact centers in the US, with over 3.5 million agent positions.
• Large contact centers (with over 250 agent positions) employ around 45% of all contact center staff, despite only accounting for less than 7% of physical contact center sites.
• 20% of businesses using a workforce management system are looking to replace it.
• Around 4% of the US’s employed population work in contact centers.
• The mean average contact center size is 85 agent positions.
• There were over 230 billion minutes of inbound calling in 2016.
• On average, 12.5% of inbound interactions to contact centers are via email.
• Outbound calling activity has declined considerably to 18%, although more service focused outbound calls are replacing past sales call volumes.