Twitter has acquired TapCommerce, a startup aimed at letting marketers “retarget” mobile users with ads to reactivate users who have already installed apps on their phones.

Also Monday, Twitter announced the expansion of mobile app-installation promotion to advertisers worldwide, under which marketers can target apps to users — via sponsored posts in their timelines — based on interest, keyword, TV viewing and other criteria.

Terms of Twitter’s deal for privately held TapCommerce were not announced. Twitter is paying around $100 million for TapCommerce, Re/code reported. According to TapCommerce, the company’s clients include eBay, Hotels.com, Gilt and game publisher Supercell.

“Advertisers spend aggressively to get new users, but reactivating existing or previous users can provide just as attractive a return on investment,” Richard Alfonsi, VP of global online sales, wrote in a blog post announcing the acquisition.

The deal comes after Twitter’s $350 million acquisition in September 2013 of MoPub, a mobile advertising network. Twitter earlier this month also acquired Namo Media, a startup specializing in native advertising for mobile devices.

TapCommerce has worked with Facebook on app re-engagement campaigns and claims it has deals reaching more than 50,000 apps, and Twitter’s acquisition “doesn’t change that,” according to Alfonsi. By the same token, MoPub continues to work with multiple partners through the MoPub Marketplace and will continue to provide “the same open and neutral access to the ad space as they have always done,” he added.

TapCommerce, based in New York City, raised $10.5 million from investors including RRE Ventures, Metamorphic Ventures, Eniac Ventures, Nextview Ventures, Bain Capital Ventures and Pereg Ventures.

U.S. smartphone users used an average number of 26.8 apps per month in the fourth quarter of 2013, up only slightly from 26.5 in the same period a year earlier, according to Nielsen — indicating there may be an upper limit to the total number of apps users are willing to access in a given month.