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.
Here are those 8 factors for argument sake:
As an example of how wrong this argument path is: right now there is a trademark application published for opposition on the word "FACE", an acronym by the Florida Autism Charter School of Excellence, Inc.
They have been using this term since 2007, and I see no reason why they will not get their trademark filed, because it does not conflict with Facebook's usage, as you simply would not get these two usages confused.
In fact, these usages are encoded into the registrations themselves: you have to specify a class and define how you use the mark, and then that mark only exists within that limited area; you can stretch it some, but those stretches are all fairly common sense.
As an example: if you had a restaurant called Apple, it would matter severely how you rendered the Apple, what kinds of other imagery and colors you used, etc.; even though this was outside the scope of computers, you might still fall under their trademark.
But, they DO NOT "own the word Apple". To continue to make such claims demonstrates a severe lack of understanding of why trademarks exist and how they work, and doing so in this post is undermining and diverting from the real issues.
Hmm, I wonder how people are going to refer to a casual, recurring sex partner now!