Hey @osvald01 - welcome to the Auth0 Community!
Could you please clarify what you mean by a social signup? Social connections are federations into Auth0 - technically, there is no signup, but only a first login, since the account is already created in the IdP you’re federating from.
The state is supposed to be used to prevent CSRF attacks, so I highly recommend using it for that rather than to return a custom parameter.
What are you trying to achieve with this custom parameter? There might be a different solution that we could use in order to achieve it.