[nsajko]

What's the point of encryption if you are going to share the keys via fb? That does not seem better than using a substitution cypher.

[ihm]

You use Diffie–Hellman. "Diffie–Hellman key exchange method allows two parties that have no prior knowledge of each other to jointly establish a shared secret key over an insecure channel." (https://en.wikipedia.org/wiki/Diffie–Hellman_key_exchange)