Facebook has recently removed the feature to download the birthday calendar.
This script will allow you to get all the birthdays in a CSV format (with some manual efforts) which you can the easily import to Google Calendar.
You can also visit https://deepakmahakale.com/facebook_birthday_export
Social Birthdays Export
Login to Facebook
Visit Facebook's events page
Scroll to the bottom of the page till you have birthdays from all the months
Right click on Today's Birthday or Upcoming Birthdays
Select Inspect from the dropdown
In html inspector right click on div with id="birthdays_content" and select "copy" > "Copy element"
Paste the content in facebook_html.txt and run the following command
python birthday_parser.pyImport the birthdays.csv generated by the program to Google Calendar
CSV Format for Google Calendar
