Could that be because the US didn't exist until the Constitution was ratified? Or perhaps the Articles of Confederation? Or at least the unified statements of the DoI made by the representatives of the various colonies?
I should have been clearer: Columbus never set foot on any land which is now nor has ever been a part of the US. Maybe the mock Facebook page should have started at the Declaration of Independence or ratification of the Constitution.
Did Columbus discover the Americas? Yes (from a European perspective, anyway). Did he land in the United States of America? No.
He wasn't the first European to arrive at the Americas. There were several before him.
He certainly popularised the area - he was an innovator in the exploitation of land and peoples, which people for centuries to come imitated:
Christopher Columbus introduced two phenomena that revolutionized race relations and transformed the modern world: the taking of land, wealth, and labor from indigenous people in the Western Hemisphere, leading to their near extermination, and the transatlantic slave trade, which created a racial underclass. (James W. Loewen, "Lies My Teacher Told Me")