I was surprised to learn that “unfriend” is a lot older than I’d expected. While it doesn’t come to be used to refer to social networking sites from 2003, there is a recorded use of it as a verb meaning to stop being friends with someone in 1659.
Defriend (first recorded use in 2004) doesn’t seem to have the same history. Could this be why unfriend became used more than defriend?
I had a little look at the Oxford English Dictionary myself and it seems that unfriend was used as a noun (and occasionally an adjective) to mean someone who isn’t a friend from a lot earlier. The first recorded use is from around 1275.
While I imagine it is the noun version rather than the verb version that’s being picked up on, I was interested to see (using Google Ngram Viewer) that ‘unfriend’ is mentioned more often in books in the late 1700s and early 1800s than it is today. And I thought it was a modern word!