Sorry, smoke signals were the first social network. Plus you couldn't even opt out of them.
They weren't even push in a way. You had to manually pull the messages. So I don't think there's a question of opt out. :)
Email's older than voice mail.
Fair enough, though I wouldn't call it a "cloud" application until webmail became prominent in the late 90s.
Why? You'd log in to servers that were more or less permanently on and connected to public internet in order to access email for the length of it's existence.