Same thing happened to Draw Something and Words with Friends. The trick is to get the 9 figure valuation, raise as much funding as possible and then put as much in your pockets before the hype dies down.
I was just thinking the same thing- HQ for me was a throwback to an era where there was a series of iPhone games that came on suddenly to the point where everyone I knew was playing constantly, and then people just kind of got bored of it.
I actually won once for something like $22, which was pretty good considering it felt like most of the time the winners got about $0.85 each.
The best way to do that is to take sizable portions of secondary sales while raising capital at high valuations. You'll walk away with tens of millions regardless of whether the app realizes an exit in the end.