Why do you see pairs of shoes hanging by the laces from power lines?
Well, I just wanted to say, around here in this student city I live in that the students usually do it for fun when drunk.
Regarding the first reason listed by Cecil, see here:
“He’s learned that a dealer throws a pair of shoes over a power line to signal that dope is for sale underneath. Now residents form teams that pull shoes off the lines.”
Note the credentials of the source:
“Topalli is an assistant professor of criminal justice at Georgia State University and a member of the school’s Partnership for Urban Health Research. But unlike most criminologists, he does little of his work in prisons. To get closer to understanding the criminal mind, he seeks out offenders whose crimes are fresh.”
Reading the whole article is more than creepy.
That’s always been kinda the main reason. Graduating from school, drunk, etc. The drug connection is rather recent. Here’s a previous thread about the subject.
Our son, at age 8 or so, was looking out the window of the car and asked, “Hey, pappa? What does it mean when you see a pair of sneakers hanging from a telephone wire like that?”
And my husband answered, “It means some kid came home with wet socks.” 
Because it’s hard to get single shoes to stay up.