I summed up the answer to this question in the below video. Please read the transcript in English below (or the subtitles) if you’re not an Arabic speaker.

1- First, we need to go to Facebook Events

2- Then go to Birthdays, as shown. Right-click, copy link address

3- Go to our Google Calendar. Click on the menu item, choose ‘From URL’

4- Paste the link, then press “Add calendar”

Now all our friends’ birthdays are on Google Calendar!