I am an MC with 5 players. Their characters, some more than others, are all EXTREMELY antisocial. Getting the characters together in the first place is a huge chore because they all need an NPC to influence them to do anything. And then, they cling to their designated NPC. When there is interaction between PCs, it usually hits a wall because either no one wants to talk to anyone, or there is nothing there.

How do I break this habit? My players want to learn about the other PCs and do all this stuff, but their characters are the most introverted, hermited people. What activities outside of plot points can I pose in the story to bring together the PCs?