I've read a bit about Line before. Crazy business model and yes, huge
I haven't looked into it at all for integration with JFBConnect, so I can't speak to that directly.
JFBConnect was built to be pretty easily extensible with new social networks. This is mainly for us to make it easier on ourselves, but we know others have added their own network integration with lesser known or more regional social networks. For basic authentication, it's a matter of creating your own /components/com_jfbconnect/libraries/provider/xyz.php file. That sets up the OAuth2 URLs that need to be called. Then, there's a similar /libraries/profile/xyz.php file that describes how to pull in profile information.
That's not to say it's 'simple', but once those files are setup, JFBConnect automatically detects them and tries to start letting you use them.
Things like Channels (for posting/reading from streams) is another level of complexity because that's usually very specific to each social network. OAuth2 is a standard.. the APIs for each social network are.. not.
I hope that gives a quick overview. If there's something specific you're looking for, or if you know more details about Line on how you need to proceed, we can gladly provide some support and assistance along the way. Generally though, it starts with setting up the OAuth2 portion to let a user authenticate and then moves into permissions and other details.
Thanks,
Alex