The Flagger app
Flagger stores your settings on your computer and does not phone home with any data about you, collect any sort of analytics, or initiate any external web requests. The app is open source, and you're free to inspect it yourself.
The Flagger web site
The Flagger website collects only anonymized analytics data through our self-hosted instance of Piwik, which is a lot more bad-ass than Google Analytics and doesn't spy on you. We anonymize IP addresses, so your traffic cannot be used to personally identify you.
That said, we do have Facebook and Twitter widgets on our page. These are third party scripts that probably do spy on you. Fuck, I'm going to have to fix that soon.
Thank you very much!