There is a whole universe of acquisition channels that can be effective for your app depending on what kind of traffic works for you / what your audience is.
Other Owned Inventory Sources / Self Attributing Networks
Snapchat, Amazon, Pinterest, Twitter, Oath, Tencent Social
Rewarded Video (SDK traffic)
The "top tier" here is Unity Ads, Applovin, Vungle, ironSource
Admob, MoPub, Applovin, Unity DSP, etc. Lots of choice here.
Tapjoy, ironSource, FreeMyApps, NativeX, etc. Again, a lot of choice.
Programmatic Content Marketing / Native Ads
Then you have a whole set of companies that offer managed out-of-home advertising / TV advertising and the associated measurement.
This isn't anything close to a comprehensive list, it's just a way of saying that there is a vast amount of choice out there beyond Facebook and Google.