I think it's too bad that the top discussion on the entire issue is the silly argument about a trademark. But since that's the case, I don't see it as a misunderstanding of how trademarks work, but a perfect way of demonstrating how fatuous Facebook's trademark claims have become. After trademarks on the words face and book now they also claim to own the letters fb. How is that any different then someone trademarking socialmediawebsiteforumblogimagegallerynews.com™ and suing the Internet out of existence. Even WordPress suggests using wp in a domain instead of the word wordpress. Facebook never had a problem with domains that used fb before, they're only complaining now as an excuse to shut down a service they don't Like™.
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.
There are actually eight different factors which the courts use as a "test" for infringement. Of those eight, I would say a majority fail the infringement test for Facebbook. Mainly because "FB" is not part of its primary trademark. Thus, the "FB" presents a weak case for the "Strength of the plaintiff's mark." element.
Your argument ignores the example, and continues to expose the same misunderstandings about trademarks that are popular on the Internet: trademarks have nothing to do with owning words or imagery, they have to do with context.
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.