Just posted a whole thread about this in Technoquette, but I would like to say that I woke up this morning to three messages in my Tumblr inbox, all of which said "Hi." and "Hey." and "So what's up?"
I've never spoken to any of these people. To me, if you want to start a conversation, wouldn't you say something besides "Hi"? I mean, if I passed a stranger on the street and they said "Hi." I would give them nothing more than a smile and a nod or maybe a "Hello" in reply. I wouldn't assume they wanted to talk to me.