What is the news here? FB was making a loss last year, so why should they pay corporation tax?
They don't make actual losses - they pay all of their profits as "license fees" to some offshore entity to get their accounting show no profits. This probably means Luxemburg or Netherlands where they have a personalized deal with the tax authorities and the taxation of immaterial property is very low.
In reality they made a shitload of money, and are using creative accounting to make it look like they made a loss. This is tax evasion, not making a loss.
This might mean they shuffled money around to avoid paying UK taxes, i.e. they engaged in https://en.wikipedia.org/wiki/Transfer_mispricing potentially breaking the law.
Most likely because the case is not so clear: it is far from clear that they engaged in any transfer mispricing.