There are rumors about a possible introduction of a metamask tokens for many many months now. In order to maybe qualify for that airdrop i used the swap function of metamask for the wallets that i am using. I would guess that you need to use that function in order to be eligible to receive those tokens. I don't know though, that is just a feeling. That being said it is not officially confirmed yet if there will be a metamask token at all.
If there will be a token airdrop then it really can be the biggest airdrop ever. That also means though that you should not expect to get a significant amount of money with that airdrop. There are just two many people that will receive it and will immediately sell their tokens.