Which part of logins and cookies is RESTful?

Fielding's REST requires that the protocol be "stateless"... not that there be no state, but that the client and the server need not keep track of each other. Cookies, as Fielding explicitly calls out, are in violation of this principle. Every website that uses cookies is not RESTful.

Some people might conclude from this that FB, Amazon, and HN are "not good", because they're not RESTful. I personally conclude something else, but YMMV.