how is it social
So a big part of this mod will be the ability to buy and decorate your own base. The social part of this will be showing off your base and going to see others'. Everyone will be participating within one cavern, so bases are going to be scattered all over, and you can check em out.
Will probably add benefits for going into other players' bases. If you go to the "important stuff" link, that shows an older topic where some detailed information about how this works was posted.
Edit: Another big part of the social interaction is the mining itself. Bigger bricks take a lot to mine them, and would be a pain to try by yourself. With a buddy you can mine these a lot quicker to yield more, and also more rare, gems.