Michael Skapinker has a great column in today’s Financial Times, “Give diligencing its due in the lexicon of 2010,” on words newly included in the New Oxford American Dictionary. (Behind the subscriber wall, alas.) He quotes Steven Pinker estimating that “a fifth of English verbs started as nouns, including ‘to progress’, to contact’, and to host’.” Moreover, even “unfriend,” as in Facebook, actually has a long history (as indeed, I would add, “un” words do in English, “unhand” or “unsex”) … in 1659, says Skapinker, Thomas Fuller wrote, “I hope, Sir, that we are not Unfriended by this Difference which hath happened betwixt us.” As for the movement from due diligence to “diligencing,” I would note that it is being marked sternly in red as I type this as misspelled. As is “unfriend.”
(PS. I agree with the commenters that “defriend” is much more the common FB expression, not “unfriend.”)