CTFU is short for “cracking the fuck up” (or, if Urban Dictionary is to be trusted, “chut the fuck up” and “chill the fuck up” (wait, seriously?)). At the Linguistic society of America meeting earlier this year, Jacob Eisenstein, Brendan O’Connor, Noah A. Smith, and Eric P. Xing presented some data [PDF] showing that CTFU was limited in geographic scope to a region between Detroit and Philadelphia.
This is a strange distribution, since it crosses a number of traditional dialect boundaries, and yet it seems to have some legs. More than a year after the set of tweets they used in their analysis, we see CTFU keeping largely to the same region, perhaps having spread a bit into the DC area. Altanta also seems to use it, but otherwise its main base of usage is from Cleveland to the NYC/DC corridor, with only a few people using it elsewhere.