A fad is popular because it's popular. A fad gives us momentary joy, and part of the joy comes in knowing that it's momentary. We enjoy a fad because our peers are into it as well.
A trend, on the other hand, satisfies a different human need. A trend gains power over time, because it's not merely part of a moment, it's a tool, a connector that will become more valuable as other people commit to engaging in it.
Confusion sets in because at the beginning, most trends gain energy with people who are happy to have fun with fads, and it's only when the fad fans fade away (yes, I just wrote 'fad fans fade') that we get to see the underlying power of the trend that's going on.