Yeah, it wasn't that helpful. In the last few weeks they made it so you can right click on a page once opened in a container and "always open in this container"
It only blocks third-party cookies, so you can login to Facebook. What it would block is Facebook tracking outside the Facebook domain.
Another option is to use something like Self-destructing cookies, which would delete the Facebook cookie when you close the tab.
Thanks for the pointer. Wish this worked on iOS, where the only option is to use a dedicated browser for accessing Facebook. Not sure how Brave deals with Facebook cookies on iOS.
https://testpilot.firefox.com/experiments/containers/