When you first sign up for Hitlist and connect it with your Facebook account, the app shows you the top cities where your Facebook friends live. With the push of a button, you can select the cities you want to visit. At that point, Hitlist can start looking out for good-priced flights to those cities.
I've been trying Hitlist since I've met the founder last year. It's really well done, a lot of care has been put into it (I really enjoy the visualization of the places I've been, compared with the places I'd like to go to).
I will do a proper review one of these days, but you should already download it, the latest version having had just hit the App Store and Google Play. Think of it as a serendipity engine: forget when you want to go, just think about the destination. Like every good journey.