I am not talking about the public graph api, i am asking is it technically possible to consume facebook's private api that facebook official apps use, and benifit from login/sign up and such features?
Like if i reverse engineered messenger or facebook app and mimic their communication with their API, whould i be able to build a working client?
Thank you.
Yes, it is theoretically possible.
However, remember that you will be easily sued if Facebook do not agree with what you're trying to do. For example, if you're trying to use their own private Intellectual Property without their permission, or if you violate their Terms of Service agreements (reverse-engineering is going to be in there without a doubt), then they can legally sue.
Facebook have public APIs with documentation which is what is intended to be used by developers. If you decide to ignore it and go digging into things which you do not own, nor have consent to access, you're going to land yourself into trouble.
I don't mean to be a kill-joy, I'm just saying how it is from a realistic point-of-view.