If I were to guess someones security question and gain access to their Facebook account is it illegal?

Or it is just there own fault for making the question easy to guess? My Facebook account has been "Hacked" not because they got into my computer but because Facebook makes it so easy for someone to gain access change the email address of the account and then just guess the security question? Is it illegal or is it my own loss?|||It is probably illegal. If I were you I would definitely not try it.