hard to see any real commercial use for this one, unless it could be used by a "revenge" product company - but even then i imagine there are lots of more suitable alternatives. Can't see you getting much past reg fee back on this one, sorry.
Yes, what safesys says is true. Furthermore, if you know birthdays of your friends, it seems more difficult to catch those of your ennemies ... thus, a technical difficulty who impediment your project, even if some people have more ennemies than friends... but meanwhile, with much effort, this project seems feasible.