To export the birthdays of your Facebook friends as an iCal file (*.ics), please login to your Facebook account in your web browser.
In the menu on the left please choose events. On the events page you will find two insignificant links at the bottom right. Upcoming events and birthdays. Please click with the right mouse button on birthdays.
A context menu opens. In the context menu select Open link in new tab.
In the address line of your browser you should now see an URL that looks something like this:
And probably no download will be started.
Please change webcal in the link to https.
Then the link should look as follows:
Now press the Return-Key while the focus is in the address line of your browser or choose refresh page.
Now a file with the ending ics will be downloaded. This is the iCal file you need for the iCal import in Birthday Hub. The file can be found in your default download directory, which is set for your browser.