"A F" is short for "as fuck". It popped into my lexicon a few years ago, when I started hearing it in Youtube videos. (See this video as an example, although this wasn't where I first heard it.)
The earliest entry in Urban Dictionary I can find is from 2011.
Looking at USA Google Trends for "A F", "A. F." and "as fuck" show "A F" has been used for something (possibly Air France?) since at least 2004, which is pre-Twitter, but post MySpace. It also shows an uptick in "as fuck" from around November 2009, which might coincide with the introduction of "a f" as an intensifier. The "a. f." line is pretty low.
This seems impossible to search for in Google Books, because A F are initials. Using the ngram viewer with "A F_ADVERB" gets no hits.
It doesn't appear in the OED online, and Greens Dictionary of Slang lumps it in with "as fuck". I'm at a loss for where to look.
Is there any evidence out there that "A F" was coined before the advent of social media? Perhaps in military slang?