And on mobile, the numbers are little better.

Even though Opera claims about 210 million Opera Mini users worldwide, Net Applications pegged the browser's share of mobile at 12% for April, just half what it was a year earlier. Most of Opera Mini's losses have gone to Apple's Safari, the default browser on the iPhone and iPad, whose owners have a voracious appetite for the Web.

(Net Applications' Irish rival, StatCounter, showed Opera with a 21.5% share in April, with Safari at 23.7%.)

That's not to say that a Facebook-owned Opera and Opera Mini wouldn't change those numbers: In the U.S., Facebook collects about one-in-every-five page views. If Facebook branded Opera and Opera Mini with its own nameplate and pitched them to its members, it could quickly boost the browsers' shares.

Opera Mini also has an edge that could play to Facebook's advantage: Apple refuses to allow third-party browsers not built atop Safari into the App Store.