Official response from Adriel Hampton
You own all your data, and we even set up each nation with its own isolated database so your data is completely separate from everyone else, and your data is never shared with anyone unless you decide to do it yourself. You can export a spreadsheet of your supporter data, donation data and survey data at any time. We believe your ownership is important and we've made the exporting process very simple.
Note that with the NationBuilder subnations feature, you can automatically share your supporter and staff data with another nation. You can read more about subnations here. If you've become a subnation and want to change that, just email us at email@example.com.