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.