And has reached 3.6 million users overnight. That's incredible! I bet the developer is kicking himself for not monetizing with ads right away, that's a lot of traffic and a lot of money.
Since the app pulls in all your Facebook friends, and people generally have a few hundred friends, I suspect the number of users of Breakup Notifier is in the tens of thousands.
[1] - Dan told me.
EDIT: (the parent comment once said 10,000, now changed.)
However CPM banners like Doubleclick combined with affiliate links to dating sites could do very well indeed.