We've just had poor luck compounded with the difficulty of marketing a game that is easily seen as just another word game...
AdMob we had CTR of about 1% and CPI of over $2.
Facebook is running an average of 1.23% CTR and CPI is running well over $3.
Millenial Media had about 0.4% CTR
Paid review on Android Headlines ($120) a little under $2 per install.
All of the install data is estimated because we can only track links. AdMob gave an actual conversion rate, but I think it was complete BS. Reason being is that we could count our installs at that time and they were saying we had like 50 installs from clicks, but for the period of the campaign we only had like 25 installs according to the Google Play developer console (our actual numbers were bigger, but the ratio was similar).
We ran a Reddit ad and that was a whopping 0.06% CTR.
Facebook has been the most successful. I think a lot of it has to do with the way that ads are presented. They look very organic and are inline with the news feed now.
Unfortunately unless you're willing to spend over $100/day you're not going to get much notice. It seems you have to expect a Cost Per Install of over $1 which is not really sustainable for most games.
At this point we're starting to focus on building games that are less intellectual than something like a word game with extra strategy. We're also working on ways to get people revved about WS when they first launch the game (training , quick matches).
The majority of the market desires short, simple games where no thinking is involved. Either that or extremely stellar games where the graphics and gameplay are so stunning that it takes tens of thousands of dollars to make it.
So we're hoping for success on some of our simpler titles which we hope will lead to more success and notice for WS. At this point we have to stop campaigning ads because they're just not effective enough