How does Facebook do it? Probably a combination of signup location and IP location information to identify US members. Further filtering by usage levels of features like messenger, statistical leaning towards those that link their phone to Facebook…
Once they’ve determined their group, its pretty simple. To use Facebook, you have to be logged in at all times. So how do they do it? They toggle a little flag in your account that says “Serve this person Facebook Watch.” When someone with the flag set logs in, they see the feature.
I would say this is more feature flagging than A/B testing. A/B testing is when you are splinting your traffic say 50/50 on a certain thing/page/text whatever on the website and based on the outcome go with that variant.
In the case above it is more about testing a feature out to a subset of users.