There is no data import from third party extensions. If you use SocialLogin the existing users will have to re-authenticate. Please note that it can be minimum hassle by enabling the option “Allow social login to non-linked accounts”. As long as they choose a login service which uses the same email address as their Joomla! user account they will be logged in and their login service will become associated with their user account without the user noticing anything different going on than routinely logging into your site. That is to say, most users will have a frictionless transition. This was put to action in a fairly large literacy non-profit in the USA a few months ago with great results.
Regarding the extension's future, yes, I do plan to keep it alive. I believe in it as a fundamental security improvement in many use cases. It helps users outsource their login experience to a third party service they use regularly, a service which has validated their identity and performs strong authentication (even when the user doesn't actually realise it). It can reduce the friction and the password reset requests to the IT desk. It's also ridiculously simple to maintain so, yeah, I think it's pretty safe to say I really do plan to keep it alive :)
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!