Three of the five are locked into operating systems: Internet Explorer, with Windows; Chrome, with ChromeOS; and Safari, with OS X and iOS.

And Firefox, while not associated with an OS maker, is backed by the non-profit Mozilla Foundation, which has used the "open Web" mantra since its inception. It's hard to believe that Mozilla would sell Firefox to Facebook, a company that has reaped billions from a self-contained ecosystem.

That leaves only Opera.

But while Opera is the one viable deal Facebook could make, the Norwegian browser comes with its own baggage: It's the fifth browser, and a distant fifth at that, in a five-browser market.

Last month, Opera accounted for just 1.6% of the world's in-use browsers, according to data from metrics company Net Applications. Opera has never cracked the 3% mark, never been in anything but fifth place on the desktop. Even No. 4 Safari has three times Opera's usage share.