the phrase, as we typically use it today, was most likely coined by George Siemens in his discriptions of connectivism,
The term ‘Personal Learning Network’ is directly derived from ‘Personal Learning Environment’, which as history shows was first used at the The Personal Learning Environments Session at a JISC/CETIS Conference in 2004.
http://www.elearnspace.org/Articles/learning_communities.htm – but it is the first time I believe I used the term “personal learning network” (2003)