If you're willing to include west coast swing, lindy, salsa, and argentine tango in "ballroom," the Minneapolis-St. Paul area has a thriving dance scene. By being open to a variety of dances, I go dancing every night of the week and often have a hard time choosing between venues.
We have 3 franchise ballroom dance studios, at least 8 independent dance studios, 5 swing/lindy clubs, a latin club, a folk dancing club, and an argentine tango club. One of the west coast swing clubs is so successful that they have their own leased space in a shopping mall, which they rent out to others in addition to their own events, and the folk dancing club actually owns their own building. The local USABDA chapter holds a monthly dance and a yearly competition, and has hosted the national championships the last couple years.