Users of the messenger service, which rolled out for Apple's iOS in August 2011, were already able to send text and video messages, even to people without a Facebook account. The new update allows, with a tap on an "I" icon, calling to a person on your contact list that won't eat into your plan's minutes as long as your are connected to Wi-Fi.

'Alternative Communications Provider'

Facebook has long denied rumors that it would eventually roll out its own smartphone, a risky move that didn't initially go well for Google with its Nexus One. The company did partner with HTC in 2008 to sell the Status, with a built-in Facebook sharing button, via AT&T .

This move lets the growing social network , under pressure to boost its volatile stock price, take another bold step into the mobile world with no risk, while further entrenching its brand as daily communication rapidly shifts from desktops to small screens. At the same time, it lays the groundwork for future, more profitable services.

"This looks to me like the mobile equivalent of what a grocer would call a 'loss leader' -- something offered cheaply or freely in hopes that responding customers will stick around and buy full-priced items," said Charles King, principal analyst at Pund-IT. "In addition, by providing users the means to circumvent their mobile-phone plans, Facebook is setting itself up as an alternative communications service provider without bearing the pain of investing in or maintaining a wireless infrastructure ."

Obviously, the feature is more appealing to those with limited calling plans.

The idea, King told us, is to bolster connection to the Messenger app -- over rivals like Apple's iChat, AOL's Instant Messenger or newcomer apps like Snapchat -- and increase access to related services and offerings. It's also a rival to Apple's FaceTime, which allows free video chatting from the iPhone via Wi-Fi.

Sweetening the Deal

"Adding a sweetener like free calling may indicate that the uptake of Messenger has been less robust than Facebook had hoped," King speculated. "In addition, the preferential treatment given to Apple -- since the service is only offered to iPhone users -- suggests there might be some behind-the-scenes tit-for-tat between the companies, but that's purely a supposition."

It remains to be seen, though, whether use of Facebook Messenger for calling will require users to share GPS data , which could then allow Facebook and Apple to target users with specially tailored, highly lucrative mobile advertisements.

Apple did not respond to an e-mail inquiry about Facebook's move in time for publication.