Facebook gives interview feedback. It reflects their core value of sharing. It's one of a few ways you can see that their process is thoughtfully and intentionally improved upon their predecessors at Google and Microsoft, and reflects the culture of information sharing that defines Facebook.

The "legal" concerns about feedback are bogus.

Google doesn't make "good/not-good" decisions. They make "hire/no-hire" decisions. Huge difference.

They will call you back in 6 months and invite you to apply again. Just memorize the basic half of the CLRS Introduction to Algorithms book, and practice coding solutions to exercises, and you'll get an offer.