Normally people would use Facebook to boost their App because Facebook has the best information about your audience's Geo, sex, age, interest.
If you want to monetize, use Google Admob if you want a reliable source of monetization
- It's backed by Google - a prestigious corporation
- It has a huge database of users worldwide
However, it is very very strict in calculating your CPC, CPI so sometimes your account gets banned because Google thinks you are "cheating" to get click, installs. Believe me, most of time, you are not cheating but still labeled "cheater" and can't withdraw your money. Also, Google know what advantage they have over other Mobile Ad Network, so they charge more from both advertisers and app publishers, which result in low eCPM. That's why people don't love admob as they are supposed to.
If you want higher eCPM, less tight banning rules, and work well in Asia Market. I would recommend Adiquity or MobGold or Adsota.
P/s: Now I am using Adsota because it offers a wider range of advertising choices for me: there is interstitial / offerwall / Banner / Video Ads.