[DanBC]

They have no authority over you.

Here's a bit more: https://www.parliament.uk/business/committees/committees-a-z...

They could find you in contempt, which would matter if you ever visited the UK.

The risk for Facebook is that the public are increasingly frustrated with big companies not paying their fair share of tax. Some people will say "but they're only paying what the law says they have to pay". Those people are failing to understand the British cultural value of "not taking the piss". Not attending would be saying "FU", and that's likely to add to the pressure to make these large companies pay more tax.