Since 2012, Facebook has wrung profits from app-install ads. This month, we saw Twitter follow Facebook's lead. Now Google is embracing app-install ads, too, in search and via YouTube.

The search giant announced on Tuesday that it will let app developers target Google's mobile customers based on the apps they have already downloaded.

"For example, if you exercise regularly and use an app to measure how far you run, you might see an ad for an app that helps you measure the foods you eat and calories consumed," Jerry Dischler, VP of product management for Google's AdWords, wrote in a blog post. As a user, you're likely to see that ad within an app or via a relevant search.

Google is also offering app-install ads on YouTube to augment its TrueView ads. Introduced in 2012, TrueView offers viewers a 5-second snippet of an ad. Advertisers only pay for the ad if consumers opt to view it in its entirety (which usually runs about 30 seconds). The ads are the same as other TrueView ads, but click through directly to a specific page within an app that you've already downloaded, rather than to a landing page.

In addition to app-install ads,Google is also offering app developers the option of retargeting users who have already downloaded apps. Such users may have the apps on their phones or tablets, but haven't used them in a while. Users can then take advantage of deep links to go directly within the app.

"Over 80% of downloaded apps are used only once and then deleted," Dischler wrote. "Lots of businesses are able to get their apps onto a device, but might never see their customer again."

App install ads have been a big hit for Facebook. The company hasn't released revenues specifically for in-app ads, but Facebook ad partner AdKnowledge estimates that up to 70% of Facebook's mobile ad revenues come from app-install ads.