all money is virtual

the only difference between them is in the amount of people who accept that money. Using € or £ to buy Farmville virtual dollars that can only be spent in Farmville is no different to using € or £ to buy Outer Mongolian <whatever> that can only be spent in Outer Mongolia.

So for FB that's real money rolling in for doping nothing more than allowing Zynga to operate games on their platform.