Facebook is expected to announce a mobile advertising network at its F8 developer conference next week that will open up the potential for billions more in mobile advertising. Google and Twitter already have similar networks in place, which industry analysts say limits the scope of what Facebook can earn. In 2013, application stores earned $26.7 billion, per Gartner. "The app business overall is not super gigantic. ... How much advertising are you going to spend to make $26 billion?" said Brian Blau, a Gartner analyst.