Facebook is now the dominant social network -- online and in mobile. It's consistently one of the top mobile sites in third party reporting about mobile usage and traffic. Its iPhone app is a huge success. So what the social site worried about?
Clearly Facebook is worried about Twitter. And it's trying to build mobile usage while trying to pre-empt Twitter as the status update vehicle of choice for PC and mobile users. Even though Twitter is still much smaller, it's much leaner and less cluttered than Facebook. I believe that Facebook does have reason to be paranoid accordingly.
Clearly Twitter couldn't replace Facebook under any scenario, in the same way that Facebook didn't and couldn't replace Google. However Twitter growth could potentially blunt or diminish a certain type of usage growth on Facebook, including into mobile. Hence Facebook's new push and nervousness about Twitter.