There are many ways you can go about planting a viral seed to world-build. You could start a story off with a character learning the viral topic. Perhaps he or she is the first. Perhaps the last. Perhaps he or she is just very heavily impacted by the news. Maybe the viral seed is spoken in whispers throughout the kingdom. Your protagonist could be a gossiping merchant spreading the news around.
For more modern settings, what goes viral may have little bearing on the world. In older settings, I think that the world-building can be great. Are the kings affairs common knowledge? Has a bard from the east become so renowned that he is cherished in a neighboring state? There are many possibilities.