Of course, zip code and birthday are a pretty huge amount of information. With the simplifying assumptions of roughly uniform distributions, knowing a birthday

Of course, zip code and birthday are a pretty huge amount of information. With the simplifying assumptions of roughly uniform distributions, knowing a birthday confers ~8.5 bits and knowing your zip code is worth ~15 bits. That's 23.8 in total. It's not surprising that those two pieces of information can pretty easily narrow down an identity. log2(3*10^8) = 28.2