A video has shown that Hackers can open the account only with the help of username. Aanand Parkash when told Facebook about this leak, Facebook after verifying in return gave it $10500.
This entirely dependent of big six digits code that was sent to the users in case they forget the password. According to Parkash on Facebook beta developer Website anyone can use this Six Digits code many time before setting the Password.
Aanand, who is a security Engineers, at Flipkart usually finds this code of required username via bruteforce. He further said that this hack was available to everyone a little while but now Facebook has done its patching to stop this leak.