After having registered for over 200 airdrops, I noticed that all of those who say "Airdrop will be delivered after the ICO", will not deliver. The ICO is over, they have the cash and the supporters may go F themselves.
The very few (can be counted with 1 hand) who deliver, actually do it before the end of the ICO.
From those who deliver 50% send such a small amount that they are dead on arrival. 35% deliver them on their own platforms where most of the times they cannot be transfered anywhere. That leaves us with 15% who actually send the tokens to an Ethereum address.
It's profitable for them, saving in marketing, it is a waste of time for you.
RE: erc-20 airdrops, how to get them. But do not buy them.