First, the query raised has so many legal facets which are hugely controversial in nature and I am afraid only superior courts will answer them.
Where you are really importer of these services itself is controversial. I am given to understand that DGCEI's view is that 'Facebook, Ireland' or its agent was supposed to get registered in India and pay taxes on the subject transaction.
Without delving upon above-controversy, my personal views are as under (i.e. Presuming that you are indeed importer of taxable service, which is liable to pay taxes under RCM):
1. In my view, your are liable to pay GST under RCM along-with interest as may be applicable.
2. In my view, you will be eligible for avail ITC against self-invoice generated now, while paying taxes under RCM.
In these forum itself, you will find that ITC availability - in these circumstances - is hotly debated & controversial topic. I would suggest you to refer to those discussions as well for your risk analysis and final decision making.
All above are strictly personal views of mine and the same should not be construed as professional advice / suggestion.