“I believe I have adequately offset this in my calculation by assuming only 10 clicks on each Birthday Link (as opposed to my projected 68).”

I think an average of 10 per user bday-wall-posts is high. You got 68, but most of the “active” users don’t have more than 50 friends. I think you need to apply your conversion rate to the average number of friends per user, than take a reduction to reflect that you’re more popular than the average user. This brings down the 324M # to, I’d say conservatively, 108M. Still, this is an 4% revenue increase for Facebook and worth pursuing, but not as lofty as you project.