This is context sensitive. As in spoken conversation, if your buddy comes up and shares a cool story with you, silently responding with a thumbs up would be rude. If you're coordinating with a team, a thumbs up is a respectful and commonly understood gesture to indicate that you approve/understand.
Very much so - you need to be careful using things like 'facepunch' (which is a very weird name for the fistbump emoji) or the pray/thanks/hi-five emoji.
