Most likely, the company would be fined and ordered to either sell off the company or agree to some other set of penalties. If they refused to pay the fine or perform their penalties, they could be banned from operating in the EU. They'd be unable to advertise in the EU, make deals with EU companies, employ people and set up datacentres in the EU, etc etc.
Companies like Facebook aren't "US based" in any way that means anything except for where their CEO normally works and which stock exchange their shares are traded on - they have operations that are central to their business all over the world, and many of those can be disrupted.
Companies like Facebook aren't "US based" in any way that means anything except for where their CEO normally works and which stock exchange their shares are traded on - they have operations that are central to their business all over the world, and many of those can be disrupted.