The only ad platform I have ever had any success with is Reddit's cheap $20 self serve ads. I haven't tried any Facebook ads though so there's that giant I have yet to try out. Reddit's great though mainly for the feedback and people who are actually interested in what you're selling and willing to critique/x post your ad.
I experimented with AdMob and JumpTap over a few months with budgets ranging between low tens to low thousands for each campaign and even for an app that was $5, it basically didn't work.
I didn't get good results with Facebook either, but maybe due to the nature of your app, it'd be better.
Ditto for Adwords.
I only dabbled slightly with Reddit and it seemed that it would show some promise if I invested more time in it.
You might also consider paid downloads promotions. Something like http://tapzilla.com/ (YC S10). The co-founders Edmond and Francis are both helpful and responsive when I run a campaign. It wasn't the right avenue for my app, but Fotobook sounds like a better fit, as a utility and at lower price point. I'm not sure if Apple's recent foray against paid downloads affects them.
