Twitter is now in public preview! It should be available as one of the identity providers in your Azure AD B2C tenant. Here are the instructions to get setup – https://docs.microsoft.com/en-us/azure/active-directory-b2c/active-directory-b2c-setup-twitter-app.
Check it out and let us know if you have any feedback!
This is currently not on our roadmap. You can retrieve this value by making a call through the Graph API. If this is needed for your scenarios, please continue voting and we will review at a later date.