Compatibility have been usung SCLogin most successfully for many years on a Joomla! 2.5 site (V2.5.28). The time has come at last to update to the Joomla! 3 series. Presently I am using an old version 4.3.3. If I update this to the latest version (V7.2.2) will iwork on the existing Joomla 2.5.28 and on Joomla! 3.8.x after the migration (via Joomla! 3.5.1).
Yes, it should. JFBConnect is still compatible with both Joomla 3.x and 2.x, though we don't test the 2.x features nearly as much anymore. I'd recommend upgrading JFBConnect on your Joomla 2.x site, then upgrading Joomla, then re-installing the latest JFBConnect once again on your 3.x site.
I hope that helps get you going, but should you need anything else, just let us know.