Facebook is expanding an advertising service aimed at developers of smartphone and tablet apps. The mobile app ads will allow companies to promote their mobile apps to Facebook's users.

Since the start of the year, more than 145 million third-party apps have been installed by Facebook users, said Deb Liu, Facebook's director of platform monetization. Roughly 8,400 advertisers used Facebook's mobile app ads in the second quarter, Liu said.

Facebook's new mobile app ads will allow the maker of a travel app to offer to promote an airfare or hotel room, for example, while a music app like Spotify could promote a band's new song. Previously, Facebook's mobile app ads were designed only to entice its users to install a third-party app on their smartphone or tablet, but not to encourage repeat visits to the app.