Trademark is fundamentally about consumer confusion. The whole point of even having trademarks is so that when I e.g. buy a fizzy drink labeled Coca-Cola, I can be reasonably assured that it was made by that company, and is not some knockoff. Similar or identical names can be used in different markets without trademark difficulty as long as there's no potential for confusion. Or such is my limited understanding.

Facebook doesn't get to own the letters "FB", but they do get to own those letters in contexts where use of those letters would confuse people into thinking that the product in question comes from Facebook. Calling your new line of leaf blowers "FB" would probably be fine, but calling your browser extension specifically targeted at facebook.com "FB" is begging for trouble.