No need to worry unless you do write a lot about your personal life or give away any details where people could identify you.

While I have FB and Twitter, nothing on LJ is remotely related to either so it would take some fascinating bit of digging to connect either account.

But, 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.