Only I can see the posts on FB page

found the solution … so for every one who faces the same problem, here is what to do:

reason for the problem:
the facebook app you created is still in developer mode.
to change this, go to the facebook developers page, open your app and go to the settings. enter your email adress here (in case you did not do before)
then go to status & review and publish your app. i. e. set the button at the top “do you want to make this app … public” to YES and confirm in the dialog box.
that’s it.

🙂