1. A method for dynamic event detection based on content from a set of social networking systems, comprising, at a computing system: receiving content from the set of social networking systems; for each of a plurality of geofences, each geofence representing a geographic region: identifying a plurality of content, generated within a predetermined time period, that is associated with the geofence; determining feature values from the plurality of content for each of a set of features; determining an event probability for the geofence based on the feature values; detecting an event within the geofence in response to the event probability exceeding a threshold event probability; for the event, determining a category probability for an event category, based on the feature values; and categorizing the detected event with the event category in response to the category probability exceeding the category probability threshold.