What does Meta's Graph API document about Facebook text?
Less than you might expect. The Graph API reference sets one text limit for Pages: the about field, shown as Description in Edit Page Info, holds 100 characters. Posts are published to a Page's feed by someone who can perform the CREATE_CONTENT task on that Page; the user feed endpoint does not allow creating posts, so tools such as bundle.social publish to Pages, not personal profiles. When the API reports tagged profiles in a post, it gives their positions in Unicode code points, which is why this page counts code points for Facebook fields.