Suppose that Facebook asks me to verify my identity. Is it then legal for me to create a fake digital image of a passport showing my pseudonym as the real name and submit that as if it were an image of a real passport?
18 U.S. Code § 1028 - Fraud and related activity in connection with identification documents, authentication features, and information
(a) Whoever, in a circumstance described in subsection (c) of this section—
(1) knowingly and without lawful authority produces an identification document, authentication feature, or a false identification document;
(c) The circumstance referred to in subsection (a) of this section is that—
(1) the identification document, authentication feature, or false identification document is or appears to be issued by or under the authority of the United States or a sponsoring entity of an event designated as a special event of national significance or the document-making implement is designed or suited for making such an identification document, authentication feature, or false identification document;
So you're 1) producing a false identification document that 2) appears to be issued by the United States.
Whether you'd actually be prosecuted for this is something that I'm not qualified to answer.