What is the best way to stay in contact with previous mentors or intern/volunteer supervisors?

It never hurts to follow the organization on social media, and to connect directly with the colleague. If this person was more of a peer, Facebook might be appropriate, but if you're not sure or they were a supervisor, try LinkedIn and keep it on a more professional basis. You can send an occasional email or message with an update about something you saw on their website or that the person has done, or some activity you have done that is relevant to them somehow. Try to customize the communication and be genuine. If you continue to have a good connection, and it's possible, make an effort to connect in real life for a coffee or something, as well. You never know what those connections will lead to.