The secret to promoting social environments are two things:
- Inconvenience
- Forced grouping and idling
When players have to sit around and do nothing 'waiting' on something as a group, they fill that time by chatting and getting to know each other.
When players can no longer quickly accomplish something and have to work towards a goal, especially if that goal is not solo'able, they become closer attached to their community.
Many MMO gamers whine about this aspect, but it's the key ingredient to what makes the pie everyone complains about missing out on.
For example, in Everquest you know what players called me in groups? They called me by my players name. You know what players call me in groups on WoW or FFXIV or any other modern mmo? They call me by my class, because I might as well be an NPC to their solo gameplay.