Over on Language Log, Geoff Pullum illustrates the (apparently important) distinction between an acronym — a word “composed of the initial letters of a phrase” such that “you can read out the initial letters as if they were a word” — and an abbreviation (the same as an acronym, except it’s not pronounceable as a word).
I agree that FTBSITTTD looks pretty unpronounceable, but then again, I would have thought that fhqwhgads is unpronounceable, but now I know that it’s (something like) [fəˈhʌg(ə)wəˌgadz]. So how about the pronunciation [fəˈtʌbsɪtəˌtʌd] — or just [fəˈtʌbsɪt] — for FTBSITTTD? It could happen.
Incidentally, note that this entry in the Urban Dictionary says that fhwqwgads is an “[a]bbreviation [note–not acronym!] of the name fhwqwgadshgnsdhjsdbkhsdbkhsdabkfabkveybvf” (listen). (If you’re really confused, you just need to see this and all will be revealed.)
The Urban Dictionary entry just cited illustrates one of many common misspellings of the original fhqwhgads, including fhgwgads, fhgwgds, and these others, just to note a few. No surprise, really. If you listen to the song, I think you’ll find almost as many distinguishable pronunciations, ranging from [fəˈhʊgwəˌgadz] (listen) to [ˌfəhəwəˈgadz] (listen).