The Social system is the most subtle of the three, but it has more to do than just spending time with other players.
This part is fairly basic, there is a stacking XP bonus for playing with other players(for the second 'M' in 'MMO'). Further the game will feature "social events" where players are encouraged to hang out and be a part of the community. We could even tie in the message boards.
Most NPCs will incorporate a basic text-parser engine that allows players to engage them in conversation. Experience will be rewarded for the use of complete sentences and proper punctuation, and NPCs should even call players out on trying to use "net speak" etc. While for the most part NPCs exist to make conversation, some will give players hints on "hidden" quests and games.
Further, players can influence the larger political game by talking to NPCs.
Conversation Mini Game
Could also create a sort of "mini game" wherein you engage an NPC in conversation and use social skills(persuasion, intimidate, diplomacy, etc) on them. This would then grant social experience points.