This is not a great defense. We know from the trail you presented that someone from FB's legal/compliance team found your extension, not the others, likely due to its visibility and popularity. That doesn't mean the other extensions are in the clear. It just means they weren't on FB's radar.
> The extension is free. There is no advertising. I don't make any profit.
I'm sorry, but that's irrelevant. The complaint wasn't that you were profiting from the ™, just that you were using it.
> If I don't know what I'm doing wrong, I can't fix it. This is not about TM infringement, but the total lack of communication by Google
I cannot disagree more. They told you what the problem was and the originating email explained that quite explicitly. You did not have permission to use the trademark Facebook (or FB). That's it. If you removed those protected trademarks, it would have been a different story.