Hello @Isabel431,
There's no facility on the Airbnb site for hosts to go directly to a guest's linked Facebook account.
The only option is to go Facebook and use its search feature to try to find the guest's page.
The linked Facebook account feature is there for the purpose of allowing eliciting and writing of references for a host or guest. It also allows the person to log in to Airbnb using their Facebook credentials. Also the simple fact that someone links the account may give a host or guest some confidence, as in the case of verified ID where the other person doesn't get to see the ID.
Steve.