As an Airdrop Hunter, first, let me answer your questions briefly -
Is it building a real community? Yes
Are these people that already join hundreds of other projects really interested in the project? Generally NO. There are exceptions anyhow
Or they would dump it in the nearest opportunity? Yes, with few exceptions
a big number would even unfollow and leave channel after getting the tokens .... Yes
Do airdrops create real communities? or just ephemeral ones? It's short living but serves the purpose.
Let me answer in detail. Generally, airdrops are meant to build a healthy community so as to attract potential big investors during the private sale, preICO & ICO stages. If you are a big investor, will you invest in a project which has only 500 members in their social media community? Definitely NO, because the community backing is very less. However, if you have around 20K+ members then you might think about investing. Because 20k members are actually gonna use the tokens without the usage, the tokens are worth nothing. So the community built is real and of course ephemeral but it serves the purpose of attracting investments. I think once the money has come in, you actually don't need a big community to follow in social media. All you have to worry is about trading volume and real-time usage of the token.
Will we dump it in the nearest opportunity? Yeah, most probably we will look for a way to dump the coins once they get listed in an exchange because we actually don't care and we don't invest a penny into them. But if some news about some project catches our eyes, we might start following the project and may hold the token or even invest in them. Few examples in my experience are Kepler, UBEX and WAB.