That's actually what got me to close my FB account. I was tired of getting emailed birthday spam because FB knew my birthday, so I changed it to April 1st and forgot about it. This April, I got about 10 or 20 'Happy Birthday!'s before I posted saying "It's not actually my birthday, April Fool's!" ... and then people kept posting happy birthday without noticing.
Wasn't worth keeping it around after that.