What would it mean for a US based company if they decided to acquire two other US based companies and US regulators approved it, but EU regulators didn't?
What would happen if the company went forward with the acquisition anyway?
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.
In theory FB could just agree with EU that they will cease all operations in EU and leave in peace, but that would effectively kill FB over long term. Competitors would crush them. Imagine a mobile carrier who can't make calls to EU or email provider who can't sent or receive calls from EU. FB is in the same situation. It's a social network. Value for the users comes from connections.
Every international company negotiates some kind of deal if they want to stay international.