If the descentralization of social networks continue, we will have to prepare for the eventual rise of the instances wars, where people will start to fight about which instance is better and which one is weird to be in and so on, but that's for the future of us all.
I'm not sure about all of them, but for Google, you can't create a new account without a valid phone number for SMS verification. If you created your account a long time ago then you're kind of grandfathered in and don't need to add a phone. They don't allow known VOIP numbers (including Google Voice) and I think you cannot use the same number for lots of accounts.
This might vary by country. My experience is with the US version of gmail.