There is a way to automate this so that the birthdays will automatically me added as you add friends in FB. It's a little convoluted but it works. This was mentioned in another thread, but the process is spread out over several posts, here's a step by step guide.
1) Sync your FB contacts to your phone
2) Sync your phone contacts with your google account, this will put your FB contacts into your google contacts.
3) now you have your FB contacts in google contacts, birthdays included. Now we have to visit Google calendar to continue. Go to www.google.com/calendar
(login to your google account if prompted.)
4) Once on the calendar website, in the upper right corner click settings
5) Once in settings, choose the calendars tab, look on the right for a link called 'Browse interesting calendars', click that
6) Click the 'More" tab.
7) At the top of the list you should see 'Contacts birthdays and events', to the right of that you will see a 'Subscribe' Link, click that.
8) on the left sidebar under 'other calendars' you should see a calendar called 'Contacts birthdays and events' if you see it, then you followed everything correctly and are ALMOST good to go.
9) Go back to the phone, open the calendars app do a manual sync, then, hit 'menu' then 'more', then 'calendars'
10 This is a list of all the calendars that can be synched with the phone, touch the checkmark next to 'Contacts birthdays and events'. Manual sync again, DONE. Birthdays will now show in the calendar!
It seems like a lot to do, but it took me about 3 minutes to set it all up. And you'll never have to do it again!