Facebook has announced that it will allow mobile app developers to advertise in mobile news feeds. "Today we're announcing a new way for mobile app developers to grow their business with an ad unit that helps them reach and reengage their users," said Facebook engineer Vijaye Raji in a blog post. Until now, Sponsored Stories, which are placed in users' news feeds, have been the only ad unit available in Facebook's mobile app. Those require a friend of a user to have mentioned the advertiser on Facebook. "In the past 30 days, we have sent people to the Apple App Store and Google Play 146 million times, via clicks from channels such as news feed, timeline, bookmarks and App Center. "Mobile ads are an additional way…