How can the gamblers verify that the actual amount of people are receiving the drop or the bonus, if there are no list to verify it? Let's discuss this.. I am curious if other people also thought about this... 
You can do it manually,by sending unsolicited email to all users if you happen to have their e-mail, or if just keep quiet and think or believe that they do airdropped the amount to the stated numbers. I would rather choose the second option, because there is no way that a regular player can have an access to the e-mails of every player of the casino.
So in your question:
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Do you think casinos are really doing these drops to 1000s of people or are they skimming from the top.... ? They might say a drop is for 2500 people, but they might only drop for 500 people and nobody will know. Yes, some of those 500 people will report that they have received the drop and it will look legit.... but is it really?
I have no comment since I really don't know if they are really true to their words and I have no way to verify if they are not if I believe the don't. Besides Casino will not give the list of players whom they sent their airdrop. So I think let us not think too much and just mind our own gameplay. It is more enjoyable than being stressed of the thinking that the casino is not true to its words on its airdrop activities. Besides what matters is that you receive the airdrop if you got qualified for the airdrop event.