I am a Mid-Westerner mostly living abroad for the last few decades but visiting the U.S. annually. I have never heard this greeting in person, but I have been struck by the frequency of it in U.S. t.v. shows. I was raised with the phrase "Hey/Hay is for horses." In other words, the word was considered rude and inappropriate except in such cases as "Hey, watch where you're going" or (less acceptable) "Hey, that's a great idea."

Looking at the material cited from the Urban Dictionary ("friendly"), it seems I have been misunderstanding a lot of these shows.

I don't think I will personally "feel" it as friendly if someone uses it on me, but I now know that I should translate it as so -- I guess.