Usually I say tee-bee-tee but I also sometimes say the French pronounciation, té-bé-té! (Probably because I'm from Quebec, aka Canada's primary French-speaking province. Either way, I think it sounds really nice.)
In my native language the closest sound to my pronunciation is "Tee-bee-tee", my pronunciation is actually "Ti-bi-ti". There are certain things that, in my mind, I keep pronouncing in my own language, like numbers and acronyms. Obviously if I speak out loud it doesn't happen, but in my mind it does LOL
Usually I just say tee-bee-tee (when referring to Bells) or The Bell Tree (when referring to the site, or when it's ambiguous), but sometimes I call it Tibet as a joke.
I say tee-bee-tee always, it’s the abbreviations I’m particular about.
TBT is the site location (ex: “here’s a sketch for art I made for a contest on TBT.”)
tbt is the site currency (ex: “I’ll make an offer of 500 tbt for that collectible.”
igb is the currency in game (sometimes I’ll also just say AC bells)
... "Tee-bee-tee" like most other folks. Although when I see "TBT" my inner dialogue will often just lengthen it out to "The Bell Tree" anyway when it's about the forums. But the currency is always just "tee-bee-tee," the short form.