Developer here: you're right that it's totally unscientific. I never really set out to do a "scientific" comparison, because being an indie developer, I really

Developer here: you're right that it's totally unscientific. I never really set out to do a "scientific" comparison, because being an indie developer, I really don't have the time or budget to spend hundreds of dollars to find out if something works or not. So I spent around the same time setting up the Facebook campaign as the AdMob one (about an hour each, including the time to make the graphic), and for the time/$ spend, the AdMob campaign trounced the FB one. In terms of conversion tracking, it was quite unscientific, true. But basically, I can track the number of new players of the game per day. Normally I get a pretty steady flow just from the Play Store. With the FB campaign running, there was no significant uptick in signups, but with the AdMob one running, the uptick in signups was immediately clear. I didn't actually know it's possible to target mobile only with FB campaigns, and so I'll definitely be giving that a go. Anyway, my point with the post was just that for someone on a tight budget, with not a lot of mental bandwidth to spare for tracking ad conversions, AdMob was just way better value for me.