From the article For Sale: Detailed Voter Profiles:
Catalist did the basic work of stitching together lists from local election officials, but some of the most valuable data came from its customers. The company described itself as a consortium, and every contract required a customer to contribute something of value back to Catalist. Rock the Vote used Catalist to identify adults it could target as part of its registration drives—in exchange, it put personal information gleaned during those drives back onto Catalist’s servers.
This is a great model for establishing a virtuous cycle. The more people that onboard and derive value from the service, the more value it is able to in turn deliver to the existing and future user base. Once this flywheel starts it just keeps going.