There are a number of legal principles that you are completely, and totally, incorrect on.
The key legal principle is causation. The poster's false statement was the direct cause of the farmer's losses. Under general tort principles, a person who makes false statements that cause foreseeable harm to another can be held liable for damages, regardless of whether they participated in the actual harmful act.
Negligent Misrepresentation: The poster could be liable for negligently spreading false information without verifying its accuracy. Even if there was no intent to harm, the law recognizes liability when someone carelessly disseminates false information that a reasonable person would know could cause harm.
Proximate Cause Doctrine: The legal reasoning follows the principle that the poster's false statement was the "but for" cause of the theft. Without the false Facebook post, hundreds of people wouldn't have descended on the farm believing they had permission to take the potatoes. The intervening acts of the potato-takers doesn't break the chain of causation because the mass theft was a foreseeable consequence of the false post.
I don't know if all of these exist in Polish Law, but they tend to hold across most western legal systems in some form or other.
To bring it down to the "its a prank, bro" defense you are invoking. The accepted counterargument is "doesn't matter if it was a prank, bro. The actions of the poster were the direct cause for the damages."
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You did invoke that the act was a prank in your original reasoning ("The original poster performed a prank..."). That actually hurts the posters case, since it points towards intention, and a reasonably foreseeable outcome.
In any event, you can absolutely be charged for a crime that you did not participate in, but that you incited in a huge number of jurisdictions.
Under many legal systems, when multiple parties contribute to a single harm, they can be held jointly liable. The poster created the initial false information, and the people who took the potatoes acted on that information. Both contributed to the farmer's loss.
Additionally, incitement is frequently a separate crime that can be charged in some jurisdictions.
to take it to the logical ends, if you posted a bounty on someone's life as "a prank", and it led to that person being murdered, you would absolutely be charged with murder in most jurisdictions.
Why do you keep presupposing that a prank is a defense? I never said it is! You keep making false accusations against me alleging that I am defending the prankster when I never was. It is you who brought up the "its a prank, bro" defense, not me.
My entire point, as I had noted too clearly, is that a prank is not theft. Despite the causation, for which there could exist disparate charges, the person cannot reasonably be charged with theft in a just world (unless additional evidence is presented). Please go back and read the top level comment by silexia which forms the root of the discussion.
I never said that the original poster cannot be charged
You literally wrote:
The original poster performed a prank, and could be found guilty of it.
"He could be found guilty of a prank" is functionally identical to "he could not be charged".
I am not
That's exactly what you did, and I quoted it at the top of this comment, in case you don't recall.