oh-sarah:

I DON’T UNDERSTAND

Do i want the tumblr application on facebook?  Does that mean every post I make on tumblr comes up on facebook? Because I don’t want this. 
I would like the choice.  Do I have the choice of linking them?

It currently does mean every tumblr post comes up on your facebook feed. If you don’t want it, don’t have it :)

For the twitter integration you can choose wether to send a post to twitter or not. I would not be surprised if in a couple of months or so there will be a tick box for facebook too.