So. Yeah... facebook eh?
You can use it to login on other websites.
Except... facebook has a bug in its authentication, which means that roughly 2% of times they don't send the required data in their response and sometimes they will send garbage instead.
But hey, it's facebook, good-old Zucky will fix it in no time, right?
Not really. Reports of this bug started back all the way to 2011. Five years now...
So, yeah. Facebook...