I don't understand how the scammers make a profit off over 80,000 likes and shares of a status giving away free First class tickets? In the long run it could harm Qantas, but as with all competitions, not everyone wins anyway.
The answer is pretty simple, when running spam or a scam, everything is a numbers game. Lets say for instance you have 80,000 likes, that gives you the ability to directly scam 80,000 people. Most probably won't send in money or provide too much in the way of personal information, but lets say that 1% will. That means that out of those 80,000 people, you'd have 800 people who would be willing to send in money or provide enough personal information to allow a scammer to obtain credit under the persons name. The hope with something as lavish as a "First Class Trip" is that a person might be willing to part with a large amount of money (say $500) in "processing fees" or a large amount of personal information. This works well for first class since most people know that First Class is both expensive and out of reach for the average traveler, the thought of passing up such a "good deal" would be hard. Another angle which might play out is the scammer asks for a credit card number since whilst the flights and room is "free", the "hotels" will need a credit card imprint.
Even if you only get 800 people to do that, you've just made $400,000 from one scam, and these are only two ways which spring to mind on how a scammer could work a collection of "likes"/
The more different types of companies you pretend to be, the wider the cross section of the community will be.
Of course, don't forget that every once in a while, the person whom has set up the fake page just likes being an coughhole, and does it purely for kicks.