You can use LoginManager.getInstance().logOut();, even if you use LoginButton because
This UI element wraps functionality available in the LoginManager.
EDIT:
Just to mention that this works for Facebook SDK v4. I don’t know if they will change it in the future.
@as batoutofhell mention, don’t forget to put FacebookSdk.sdkInitialize(getApplicationContext()); to initialize the facebook sdk. Please see here for the details.