It's not even a threat really. Everyone is quoting an SEC filing by Facebook where it lays that out as a worst-case scenario - which is what companies do so they don't get sued by stockholders trying to get them to pay up because they "hid information" from them if there is any further trouble with EU access (which there likely will be). Facebook just has officially told everybody the obvious fact of "we have issues with EU regulations, this could get bad", as they are supposed to do as a public company.