Facebook ready to take on mobile ad network?

Facebook may finally be ready to get into the mobile ad space.

The social-networking giant plans a version of Sponsored Stories that would appear in mobile users' News Feeds by the end of March, sources "with knowledge of the matter" told Bloomberg.

The company had been expected to roll out a mobile advertising service earlier this year, and this planned foray might still be shelved as well, Bloomberg's sources cautioned.

Facebook representatives declined to comment on the report.

Despite being late to the party, Facebook already has access to data of a large mobile user base that could help advertisers more accurately target potential customers than a mobile browser or app. Facebook says nearly half of its nearly 800 million users already log in via mobile devices, giving the network momentum in a market estimated to be worth $630 million.

However, Facebook will have to contend with Google's Admob, Millennial Media's ad network, and Apple's iAd, the latter of which has reportedly run into obstacles that forced it to offer more attractive terms to advertisers. But the fact that Facebook's mobile apps are already among the most popular on Google and Apple's mobile platforms should resonate with advertisers.

The effort could bring more cash to company coffers as it plans its IPO. Facebook, which is already expected to record $4.27 billion for fiscal 2011, is widely expected to go public between April and June 2012 with a $10 billion offering that values the company at $100 billion.