If a user of a social media app, publicly posts content incriminating themselves, may police use that as evidence against the individual?

No$,$ because it would violate the wiretapping provisions of the ECPA.

Yes, because there was no reasonable expectation of privacy in the post.

No$,$ unless the user granted the police permission to access the post.

No$,$ because users have an expectation of privacy in social media content.