It's not ridiculous. I'd say if US and UK policy on privacy continues to diverge it may become necessary or even obvious.
It only seems ridiculous because it hasn't happened yet.
Other solutions can be found to keeping in contact with friends that move away without having to give undue power to some international tech behemoth. And while building a new Facebook wouldn't be easy, I wouldn't say it would be necessarily hard either. It'll just take time. Hell, the competition may even be good for Facebook on the end.