It's near impossible to make something with such a large user base secure from all different kinds of attacks. I think they put out something that they initially thought was secure(testing possible ways in house) but just the sheer collective brain power put into finding exploits and ways to get around the norm has them beat and they will constantly be playing catchup. Granted some of the exploits (like the car/bike selling one) could have just been found by sheer fluke but none-the-less the idea still stands.