Flo was charged by the FTC with deceptive data practices and as part of a settlement had to demand that its partners including Facebook remove the data shared without permission. Would you be open to discussing your concern about this for a story I'm working on? I'll reach out to you via chat now.
As for alternatives, there are lots of basic apps out there that are free to use and don't require people to submit a lot of info. I think it really depends on what you want out of these sorts of apps. Personally, I use one called P tracker and find that it's accurate despite a fluctuating cycle, and I only tell it when I've started and stopped. https://play.google.com/store/apps/details?id=com.period.tracker.lite&hl=en\_US&gl=US