Confidentiality
Most companies, and I presume Virgin included, will often have a confidentiality clause in the employees' contract of employment saying something along the lines of the 'company's affairs remain private and should not be discussed elsewhere, especially with third parties'.
The Virgin employees have 'gobbed' off, be it a whinge or a moan. Unfortunately for them, they do so in writing which provides ample hard evidence in any disciplinary hearing of any alleged misdemeanour. If it was a conversation which was overheard, they could have argued that it was misinterpreted, hearsey, etc, and possibly got a severe reprimand. Not knowing the inner workings of Facebook, I can only assume they didn't cover their tracks too well with regard to their real identities.
Moral of the story, especially in the current economic climate, keep your mouth shut and be careful where you vent your feelings.