"...if you are concerned you can add a bit of code which removes the choice for FB or Twitter commenting.

In Journal menu -> Journal Style. -> Customize Theme -> in the Left navigation bar choose Custom CSS -> insert ".b-repost-item {display:none}" minus the quotation marks. "

It'll remove the instant "repost to facebook/twitter" option, but it won't remove the option for people to log in and answer comments under their Facebook/twitter usernames.