The two companies have held multiple rounds of discussions, according to people with knowledge of the talks. But they have yet to find a compelling way to collaborate, perhaps because their courtship got off to a rocky start. Last year Facebook rebuffed Apple's attempt to connect Ping, a new social network built around iTunes, with Facebook, purportedly for technical reasons. It was a rare public rebuke for Apple, and Steve Jobs personally called some reporters to voice his displeasure. That Apple chose to bake Twitter, not Facebook, into the most recent version of its mobile operating system has not helped. Still, the two companies continue to talk, knowing full well that an alliance could help them fend off a common enemy.