The WT has always warned about gatherings of JWs where there is no control, be it JW roller skating, or big get-togethers of any sort, they are against it if they can't control it. They are after all a mind "Control" cult.
Face book is a very real threat to the WT's control over the rank and file and it has the very great potential to undermine that control.
They are panicking over the fact that the Disfellowshipping process used to keep the average JW in the dark about why so many are leaving the JWs will be rendered ineffective by anonymous contact with former JWs and by those who are still JW but remain anonymous for sake of family contact will have an avenue of contact that supersedes or bypasses their control restrictions on who the JW can communicate with.
The WT is terrified, expect much more in the way of warnings and even disciplinary actions for those not heeding the WT's Face book warning in the future.