Publicize currently sends the first few lines of your post to Facebook. If your post is quite short, that can indeed mean that the whole post content ends up on Facebook.
We’re considering adding more options to Publicize, as mentioned here:
https://github.com/Automattic/jetpack/issues/261
Until we make progress on this, I’d suggest manually editing the Publicize message for each one of your posts, thanks to the Publicize settings box appearing right above the Publish button in the post editor.
I know that’s not ideal, but as soon as you set a custom message, Publicize will stop sending a post excerpt to Facebook.
