This is a relatively new tradition, and in practice, it is more often men sending men pictures of their own chest, or men convincing random girls to send pictures of their cleavage to their friend on his birthday. However, the act was originally conceived to span an influx of casual acquaintances of the opposite sex to send you risque pictures.
Man your phone has been blowing up all day. I hope at least some of them are Birthday Boobies pics.
Well, I did get these from your sister...
How did you get pictures of Lisa's tits!
Andy really came through at the bar last night. He must have had over 40 girls send me birthday boobies. At least a dozen of them showed nipple.