Twitter has been testing a new type of ad in your Timeline for a few months now, and it's been such a hit for the network that you'll be seeing a whole lot more of them. The mobile app install ads should look familiar--Facebook has been raking in money from similar types of ads in your News Feed.
The new ads show up in your main stream sandwiched between other tweets as promoted tweets compelling you to download an app. Twitter tested the install ads with brands like Lyft, Dots, and EA, with each company touting the new channel as a cheap way to grow download rates. Armed with those case studies, the network is opening up app install ads to all advertisers.
The Twitter app for iOS also notifies you when the app you've installed is ready to open, which drives you away from your Timeline into an advertiser's waiting arms. Twitter only makes money when a user taps the install ad to open the App Store or Google Play, or when an app is opened from Twitter, so it's in the network's interest to kick you out of Twitter and into another app.
Twitter's move toward a lucrative form of advertising isn't at all surprising, given how well the format has worked for Facebook. Twitter is also using targeting, much like Facebook, to tailor ads to your preferences--so you won't see an ad for an app you already have, for example, or an app that's far outside your interests. What a relief.