It's not impossible, really. It happened before, too. Like Yahoo it won't disappear overnight, but it may easily become 2nd tier in a matter of months. Friendster and MySpace had that. Internet Explorer seemed unstoppable before. Also nobody predicted Apple's market cap to surpass MS+Dell+HP combined. Internet/Technology market is highly fluid. Social networking even more so.

As for cars... do you know why hairdressers try not to cut too short? They want you back soon. Carmakers are happy to see more integrate with Facebook now and change it for Twitter in 2-3 years if that helps to move the new models.