Why Facebook has no font menu
Facebook renders every post, comment and message in the system font of whatever device is reading it. There is no bold button in the post box and no font picker, because Facebook leaves typography to the phone or computer.
What this generator produces is not a font. It swaps each letter for a different Unicode character that looks like a bold, italic or script letter. Those characters are real text, so they paste into Facebook like any other text and look the same to every reader whose device has the glyph.
The trade-off is that they are no longer the plain letters a, b, c. Facebook search, screen readers and translation tools treat them as symbols, which is why styled text works best for emphasis rather than for whole paragraphs.
Using styled text in Facebook posts, groups and Pages
Posts: a bold first line works like a headline in the feed and gives a long post a clear start. Keep the rest plain so it reads and translates normally.
Page posts and ads: keep product names, prices and calls to action plain, since that is the text people search for and copy. Use one styled line for a section title or a short offer.
Groups: many groups use bold labels such as "Question" or "Update" at the top of a post so it stands out in the group feed. Check the group rules, because some admins ask members to avoid fancy text.
Comments: a short bold word can make a reply stand out in a busy thread. A whole comment in script mostly makes it harder to read.
Your intro, your name and Messenger
Your profile intro allows 101 characters and accepts styled text, so one bold or small caps phrase there is a simple way to make the profile look finished. Styled letters count toward the 101 like any other character.
Your profile name is the exception. Facebook's name standards reject symbols and unusual characters, so save styled names for Pages, where the name rules are different, and test it before you rely on it.
In Messenger, styled text sends like normal text. It is handy for a quick bold heading in a group chat, but avoid it for addresses, codes or anything the other person needs to copy and use.