Facebook acknowledged in a pre-IPO filing with the U.S. Securities and Exchange Commission that its "daily active users [are] increasing more rapidly than the increase in the number of ads delivered."

In the filing, Facebook listed mobility as a "risk factor" for investors, and admitted that it had no way yet to monetize the mobile users.

"Facebook definitely has to do something to get some value out of their mobile user base," said Dan Olds, an analyst at Gabriel Consulting Group. "Facebook users are increasingly using smartphones to access their pages and, until now, Facebook hasn't had much success in getting ads in front of those eyeballs."

He also pointed out that the company has to walk a fine line to keep mobile ads from being too intrusive, which could drive users to other networks.

"This is a double-edged sword," Olds said. "If the [ad] frequency is too high, users will protest and, over time, will use Facebook less on mobile devices. If it's not frequent enough, then there might not be enough impressions to make it worth the money for advertisers."