Feels cliché at this point but should be said: I am not a lawyer and this is not legal advice.
This sort of thing is about risk calculation and mitigation. How much risk is there, and is there anything you can do to minimize yours?
In this case, let's consider the context. Facebook needs to have a means to throw the book at someone who starts a site called "Phacebook" and uses a white 'ph' set in Klavika on a blue background as its logo. If some big company puts out a social media icon that's similar to the FB logo but different in some key ways and it's gaining a lot of traction, they need to have the means to shut it down. And so on.
Meanwhile, Facebook benefits from as much exposure as possible. They want every site to have a Facebook page, and they want you to drive traffic to it. I know that if I used a solid black "f" logo to match the color scheme on my site and I got a legal notification to change it to an official asset, I'm more likely to remove my link altogether than change it. They don't want that, and I haven't heard of a single example of them actually doing that.
So, this ultimately comes down to a "letter of the law" vs. "spirit of the law" thing. I see sites for companies with way more lawyers than I'll ever have who are technically violating the guidelines. If I see/hear about Facebook serving companies for improper use of FB assets, I'll see what they're looking for, what happens in those cases, and act accordingly. I'd certainly be timid about starting an "App Store" after Apple went after numerous companies that used the name. But the de facto order of things is that this isn't a big deal, and so I'll personally treat it that way until I see or hear otherwise.