That’s a tough one! I haven’t had much experience working with actors with agents, since all of my projects so far have been very low budget and largely volunteer. Perhaps @HackettKate can shed more light on that level?
As to your other more general concerns, verbal agreements are rough, because you really have no recourse if the person doesn’t hold up their end. We actually have two great articles about writing contracts for beginners, here and here, if you’d like to brush up! Feel free to comment with further questions on either if you have follow ups!
Do you not have a phone number for the actor? My general rule is to contact them via email (or, in your case, FB) several times over the course of either a few days or a week, depending on your timeline, then eventually just call them directly, because it’s much harder to avoid that. If they still don’t respond and you have to move forward, call and leave a voicemail that you have to part ways due to lack of communication. With just a verbal agreement, that’s really all you owe him, and even if he eventually gets back to you, this kind of behavior can’t be normalized by you giving him so many chances- it’ll just get harder once you start filming and have to decide between reshoots or dragging out the production to fit his whims.
Hope this helps!