Many eyebrows were raised when Facebook rolled out Sponsored Stories in January of 2011, but two years later the once controversial ad format is paying off in a major way. 

During an earnings call earlier today with investors and analysts, Facebook revealed that it makes $4 million a day from the ads that appear in user's News Feed. That's four-times what they were bringing in a year ago. 

Sponsored Stories, similar to Twitter's "Promoted Tweets," are advertisements that take the form of a Facebook News Feed post. The brands target users that have had some sort of interaction with it either by Liking its page or visiting and checking in onto one its establishments using the social network. 

More interesting is that three-fourths of that $4 million comes from mobile usage. During the call, CEO Mark Zuckerberg said that people who used the mobile app or accessed Facebook via the mobile web spent more time on the social network. Mobile's been a sore topic for the one-billion-strong network, but this quarter shows it's moving in the right direction. 

[via VentureBeat, Forbes]