The largest social structure will be the Kingdom - a player-run nation consisting of two or more Settlements. Creating Kingdoms will unlock the most exotic and powerful structures, and those structures in turn will enable characters to learn and use the most specialized and unique character skill and abilities and to craft the most unusual items.
Kingdoms will have thousands of members - the members of all the constituent Settlements. They'll produce the largest armed forces, with the most potent siege engines. Kingdoms will struggle with each other for control of vast swathes of territory. Their interactions will become the legendary stories of Pathfinder Online.