The Quora statement that the word "facebook" cannot be used in a domain name without permission is incorrect, or at least too broad. If a site or app is intended to work with Facebook, it may say so. However it may not use the word "Facebook" whch is trademark, in such a way as to suggest that it is an official Facebook product, or that it is endorsed, authorized, or sponsored by the makers of Facebook, or to cause confusion with the original Facebook. Whether a domain name containing "facebook" does that depends on the details. There are multiple precedents for criticism sites. A domain such as "facebooksucks.com" would not be trademark infringement, and there is caselaw to support this. But that is not what might be wanted here. Perhaps "unofficialfacebookhelper.com" would suffice.
But to return to the question asked, use of a word in the hypertext metadata in the "Head" element is a reasonable way of indicating that the site is to be used in connection with Facebook, but there would need to be some additional notice or disclaimer making it very clear to any user that the site is in no way affiliated, endorsed, authorized, or sponsored by the makers of Facebook.
The use of a trademark to identify another product with which a given product works is known as nominative use. The law specifically allows it, providing it is not done in such a way as to cause confusion, or to falsely suggest sponsorship, approval, or some relationship that does not exist.
However, large corporate trademark owners are known to claim more rights than the law allows them. Such owners may try to suppress nominative use that they hae not approved, although they could not win a trademark lawsuit over such use.
15 U.S. Code § 1125 (c)(3) provides the relevant part:
The following shall not be actionable as dilution by blurring or dilution by tarnishment under this subsection:
(A) Any fair use, including a nominative or descriptive fair use, or facilitation of such fair use, of a famous mark by another person other than as a designation of source for the person’s own goods or services