How does Facebook assign session keys to users?

If the user is login on PC A, and login PC B, also login in Mobile C, should there have three different session key to maintain three connections?? or there is same session key for these three device.... ... Once the user change the passw开发者_高级运维ord, just clear all the session, and make them login again?

Session keys are assigned to each pc / mobile device. So in your scenario, there are 3 session keys. If the user changes the password, it would be a good idea to clear the sessions and re-login.