One big reason that investors like Facebook again: The company looks like it has figured out how to make money — a lot of money — on mobile. One big reason Facebook looks like it has figured out mobile: App-install ads, which developers use to encourage Facebook users to download their software onto their phones. Now Facebook is going back to the well, and will try to make even more money from app developers. This time it is selling them ads that link directly from Facebook’s app to apps users have already installed, in hopes of driving usage and/or sales. The idea: A music app like Spotify could use the new ads to direct listeners to new songs that have appeared on the service. Or a retailer could tell a potential shopper about a sale, and send them directly to their app.