SDN is a model. OpenFlow is an API.
Another helpful article this week by Lori MacVittie explained that SDN and OpenFlow are not interchangeable terms. Although OpenFlow is a popular standard for implementing SDN architectures, there are alternative standards or commercial APIs that companies have created for their own solutions that will still yield a software-defined network.
In addition to information on OpenFlow's relationship to SDN, the article also has some small survey results from Information week showing how familiar people are with OpenFlow from 2012 to 2013.