const Goa = imports.gi.Goa;
const Gio = imports.gi.Gio;
const GFBGraph = imports.gi.GFBGraph;

let client = Goa.Client.new_sync(null);
let accounts = client.get_accounts();
let authorizer = null;

for (let i in accounts) {
    let account = accounts[i].get_account();
    if (account.provider_type === "facebook") {
        //authorizer = new GFBGraph.GoaAuthorizer(/* what? */);
        break;
    }
}

...