A screenshot in news suggests otherwise - 'we'll figure out who you are' is not something I want to see by these tickyboxes! Since LJ are in possession of many email addresses, some of which are going to be the same ones used for Facebook and Twitter accounts, it is far from impossible for them to connect accounts. (My Twitter account, for instance, has this user name, and the same email address. I shall be changing that, as it's trivial to do so.)