At the moment we're witnessing a fight for internet superiority through ecosystems. Facebook see Google as their major competition in social networking, but that's just a feature of the ecosystem. If another company can leverage one of their strengths and use it to push into social networking (Android Google+ integration), Facebook won't be happy.
Amazon are in, Google are in, Apple are in, Microsoft are in, Samsung aren't far away.
Amazon, Google, Microsoft, and Facebook all come from a strong software background, but are having to push into hardware, whereas Samsung are coming from the hardware side, and Apple are perhaps best placed in terms of experience in both mobile hardware and software, though historically their cloud services have been lacking.
At the moment, Facebook has a place in all of these ecosystems as a feature simply because of its user base, but google has its sights set on Facebook, and will try to steer users towards G+ with Android, and if Facebook starts bleeding active users, it's in trouble.
So what does Facebook do? I'd guess that they'll follow Amazon (and likely Samsung) in forking Android, and use Microsoft services (search, maps, mail) to fill in any gaps they have, and aim at the low end of the market, hoping for quick adoption with teenagers.
Pure speculation, of course.