Apple's new Mountain Lion OS, which was previewed Thursday, includes a sharing feature readily available in most apps -- but users can't use the menu to share via Facebook.

The "Share Sheets" function is one of several elements that Apple has dragged to the desktop experience from its iPhone iOS. Like iOS 5, Mountain Lion makes it easy for users to share via Twitter across a number of different Apple applications once they've enabled the service and logged into their Twitter accounts.

But with more than eight times as many active users as Twitter, Facebook is "conspicuously absent" from the sharing menu, said Gartner analyst Ray Valdes.

With no apparent technical barriers to the inclusion, analysts said, the case must be one of stalled negotiations.

"Probably there is some substantial issue having to do with revenue, ownership or business model," Valdes said.

OriginalURLIn an indicator that the Jobs era is indeed over, Valdes and Roger Kay, president of Endpoint Technologies Associates, speculated that, behind closed doors, it's more likely that Facebook is playing hard to get than that Apple is shutting out a perceived competitor.

"Facebook is in the business of trying to get people to use its entry points," Kay explained. Appearing on Apple's menu would allow Apple to "own that piece of the perimeter."

Jeremiah Owyang, an industry analyst at Altimeter Group, disagreed. "Facebook wants to be everywhere consumers are," he said. CEO Mark Zuckerberg "has said he wants to capture everyone's lives into the open graph."

Of course, Facebook also aspires to keep users inside its expanding perimeter, even when they stream music. Owyang thinks that makes Facebook look like a growing threat to Apple, and "Apple has been really defensive in drawing moats around its media revenue."

The real cause will likely become evident in future updates to Apple operating systems, Kay said. If it's just a question of finding mutually agreeable terms, as he and Valdes suggest, an update will likely cause Facebook's blue F to pop up in the sharing menu. If the social network's exclusion marks the beginning of a serious corporate rivalry, that will also become clear.

The companies aren't tipping their hands yet. Apple did not respond to requests to comment. A Facebook representative said, "iOS is an important platform for Facebook and we have a good relationship with Apple." But, she said, "we don't comment on what we might or might not do in the future."

Readers can find a full overview of the Mountain Lion OS here.