My advice is to ask people how they would like to be followed up with. As you stated in your question, people prefer to respond to emails. If you are finding that to be true, then that's how you want to stay in touch.
That said, I've always coached that the order of most effective to least effective (for most people) is: Best- face to face, then- phone, and least- email. The reason being is because of the way humans communicate. We develop stronger rapport in person (how many ended friendships started with a misread emoticon in an email?).
Communication is 7% the words, 38% your tonality (how you say the words), 55% physiology. In an email, all you have are the words, so the majority of communication is lost and can be misunderstood.
Respect your contacts' wishes, and when ever possible, go face to face or phone.
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