If I haven’t met an affiliate in person before and never been in touch, it would have to be email, forum msg or messenger.
If I met an affiliate on the conference and we had a great time together (at EuroPub in Amsterdam for example 
then phone calls seem to be a natural choice, although messenger is also a great tool, especially if we just keep in touch and there’s no need for any major updates.
I am aware that some of the affiliates keep their full-time jobs or have other responsibilities and are not always available 9-5… that’s why I usually try to schedule a call beforehand. With messenger things are way easier, as you can see when the other person is online 
Email always good for follow-ups, but you guys mentioned that already…