by Arturs Polis.
Thank you for suggestions, Ken!
The debugging mode did not show any problems...
Caches purged...
I also tried out with Google / Facebook alone...
Nothing! - it does not show up... Could it be because our Moodle course is on a sandbox website protected by a password? Will the buttons appear as soon as we move the course to a public site?
What other things could I try? (or what should I ask to my server administrator?)