There's 7.6 billion people in the world. That's a substantially larger "US" if you are going to reframe the OPs comments as implying "US vs Them."

Also given the recent velocity and volume of bad PR for Facebook, it's possible that public sentiment is irreparably turning against them.

Lastly, technology has no shortage of once popular products that have been relegated to the dustbin of history - see AOL, Nokia, Friendster, etc. Today's dominance is far from a guarantee.