In the original 4.3.3 loginregister.php file, the line you mention is actually at line #300. What changes have you made to that file, or that function, as it seems like there's at least an extra 100 lines between the original and your file.
As for the problem, the loginregister.php model file defines the getBlankUser function. That code was taken directly from the Joomla - User plugin code to get an empty user with all the defaults. The function that is calling that (createFacebookOnlyUser) is called in the login() function of the same file. There are multiple checks to make sure that the user was properly created and there shouldn't be any corruption that should occur if something goes wrong. The checks should catch odd behavior, and even if they don't, the actual Joomla login call shouldn't work, which will prevent the session from being updated.
I'm not sure what's happening, but we'd definitely need to know about any modifications you've made to the files and, if possible, determine what's happening to the session table. With that, we'd at least have a better idea of the cause and could hopefully help you investigate.
Thanks,
Alex