Mobile user acquisition is the act of bringing new customers to your brand and into your app. Without acquisition, you have no customers. However, it’s not as simple as setting up a website and waiting for the users to come flooding in.
To be blunt: acquisition is expensive.
Depending on which source you look at, it costs anywhere from 5x to 16x more to build a long-term business relationship with a new customer than it is to simply cultivate the loyalty of an existing customer. And when you factor in that the average Cost Per Install (CPI) in the US is $2.07 for iOS and $1.72 for Android, you’ll realize what a huge cost is involved. 
Which is why it’s imperative that you have a user engagement strategy in place once acquisition is done.
Because of the cost of acquisition, the hurdle remains: how do you acquire the most customers using the least amount of money (and the least amount of work)? The answer will determine whether you get a good ROI from your acquisition efforts.
As with any marketing campaign, the goal is to raise brand awareness and lead prospects down a path to becoming brand evangelists. But there really is no single formula.
Instead, marketers use a combination of the various marketing channels to drive people towards their app, these channels include:
In the world of mobile, user acquisition is measured by two metrics: number of installs as well as install rate.
In addition, there several other measurements that talk about the cost of acquisition, among them: