It may be easy to send a message to a few dozen or a few hundred of your contacts in bulk on LinkedIn (or anywhere else for that matter) but unless those you send it to know you well and respect your correspondence, you're probably killing the messenger and the possibility of any of your messages being read when you do.
Personally, it doesn't matter if you start it with Hi, Hello, Dear Friend, etc. If my name doesn't follow you have less that a one percent chance that I'll read it; and I'm guessing I'm not the only one who treats them that way.
