How Many of Your Facebook Friends Know Your Actual Birthday?

Outside of my immediate family, I have no idea when anyone's birthday is without the aid of Facebook. And you probably don't either. David Plotz at Slate wanted to see what would happen if he celebrated three birthdays in July.

The results were predictable. He got a bunch of wall posts for each of his July birthdays (his actual birthday is in January), with one industrious fellow wishing him a total of four "HBDs."

So what about you, dear readers: how many of your Facebook friends would notice something was amiss if you started celebrating birthdays every few weeks, half a year removed from your actual birthday? [Slate]