My sense is that re-targeting is something that most -- maybe 75-80%? -- mobile advertisers have not experimented with. When I ask clients why they aren't currently running re-targeting campaigns or have never run re-targeting campaigns, the most common response is: it doesn't make intuitive sense that a user that churned should be advertised to again.
This is sensible logic, but I've also seen enough successful re-targeting campaigns to know that it isn't absolutely true. For one, an app can have substantially changed since a user churned, offering new functionality and purpose that is appealing. And second, many apps are intent-driven (eg. travel, dating), and "churn" can be temporary for these apps.
I've seen a broad range of re-targeting utilization, from situations in which most of the advertiser's spend is on re-targeting campaigns to (more commonly) situations where the advertiser isn't running re-targeting campaigns at all. Here are a few thoughts:
First, it's important to distinguish between re-targeting and re-engagement. Re-targeting campaigns are generally targeted to users that have either churned or who expressed some interest in the app (eg. clicked an ad) but didn't ultimately download. The purpose of these campaigns is to engage people who aren't currently users of the app.
Re-engagement campaigns are targeted to existing users of the app for the purposes of increasing their participation in the app; for instance, a re-engagement campaign might be targeted to existing users of a mobile game around a weekly event in an attempt to get them involved in it. Both of these campaigns should be designed to create incremental increases in engagement / monetization, meaning new revenue is generated that wouldn't exist in the absense of these campaigns.
What does that incremental engagement / monetization look like? For re-engagement campaigns, it means the existing, active user sees an ad and performs some action as a result that they otherwise would not have been expected to perform. For re-targeting campaigns, it means a non-user sees an ad, downloads the app, and engages or monetizes in a way that they otherwise wouldn't have.
Since the idea behind incrementality is that only new revenue is attributed to the source campaign, the most common form of measurement is through holdout groups: taking some subset of the targetable audience, setting it aside, and not exposing ads to it so as to establish a baseline of performance. How the holdout group behaves is meant to represent the negative scenario -- what would have happened had the ad campaign never been run. By comparing the holdout group's performance to the targeted group's, the advertiser can assess the impact of its campaign and the degree to which it delivered incremental revenue. A typical size of a holdout group is 10% of the overall targetable population, although this might be lower if the targetable population is very large.
The companies that I've seen rely on re-targeting and re-engagement campaigns as an important / critical source of new revenue generally run them constantly, using rolling churn definitions (eg. "users that haven't logged in for the last 30 days") to continuously create "lists" of users that they can target on advertising platforms. All of these campaigns utilize holdout groups for measurement and then the holdout groups are targeted in the next "round" of campaigns so as to not reduce the impact of re-targeting / re-engagement by 10%. Since this measurement work is so consistent, these advertisers usually have dedicated analytics infrastructure and tools to help with analysis.
The most common platform that I've seen used for re-targeting is Facebook, since Facebook makes uploading audience lists via API very easy. I've also seen re-targeting DSPs, like Remerge and Adikteev, be used to facilitate re-targeting and re-engagement campaigns. These companies take lists of users from advertisers and bid on inventory as they "see" those users in auctions on programmatic exchanges.