If they take my package, never deliver it, and claim they delivered it, that's theft of tangible property, punishable by state law (and federal, if they messed with interstate commerce).
There is no such "theft of information" for failing to route a data packet, nor should there be for obvious reasons.
Better analogy would be if a newspaper took money to run my ad but never ran it. Also theft, or at least try-able in civil court for failure to render service.
... But if they take a message from me, for free, on the understanding they'll route it and never route it? Much, much grayer legally. I have to prove harm and that they had a legal duty they abrogated. Neither are obvious regarding Facebook DMs.
OK, what about letters instead of parcels? It's not for free either, it's paid for by the metadata. They ask for a recipient, you tell them the recipient and they say ok, will deliver, but never do.
> It's not free either, it's paid for by the metadata
That's one of the things that could be argued in a court of law... That it was wrong for Facebook to collect data from someone on the false pretense that they would send a message they would not send. I wish the plaintiff who argues it the best of luck... Hell of an uphill battle. The notion data about a person has tangible value that belongs by default to the person and can therefore be exchanged is an open question (legally and morally). And there's still the issue of proof of harm. In fact, that's the biggest issue.
If a person thought they had communicated something via Facebook and Facebook falsely claimed it was sent when it was not, and that person could prove material harm as a result, they might have a civil case to rectify the specific harm. I struggle to imagine such a scenario, but I'll assume it could happen for the sake of argument. "Facebook silently black-holes your attempts to contact a celebrity" as an example, is not harm.
They take parcels, give tracking numbers, but those specific persons never receive anything. Tracking is always stuck on "parcel enroute".
edit: clarified wording, to make it clear that fedex shadowban is a hypothetical situation