For me a like is an acknowledgement that I have read a note and I appreciate their time in writing it and their thoughts on the topic. A zap is for a note that has evoked an emotion, any emotion. Zap is also for those notes that have made me stop and consider something, or has taught me something new, or has promoted a thought/cause I believe in.
Discussion
I like that way of looking at it. If it evokes an emotion, ⚡. If you appreciate the time but it didn't evoke anything, ❤️. Good rule of thumb.