There are many things that might have been done; if the warrant should not have been served, the best person to challenge it would probably be the defendant (or

There are many things that might have been done; if the warrant should not have been served, the best person to challenge it would probably be the defendant (or their counsel). They would likely challenge the admissibility of the evidence. I am not sure what grounds Facebook would have to challenge this warrant, unless it would be improper for them to provide private communications for some reason.