why we need to share something on posts from this forum on facebook or twitter?
i don't think its a good idea but login/signup from social accounts is what i want to see in new forum software
The idea of the OP is good, what i would add is to be added a share button on each post, so we can share it directly and others can see the post instead of the thread.
Also your idea to login/signup is a good idea but the security is not good enough, how to reset the password with them when the email address in facebook has not been used for many years or lost password, that's the worst case.