I've noticed that most people start off emails with "Hi Bob,". I suspect they're treating "hi" as a less endearing replacement for "dear" in "Dear Bob,". Technically, "dear" is an adjective describing "Bob" while "hi" stands on its own and is not part of the direct address. I prefer the more correct salutation: "Hi, Bob."
Who is the most grammatically correct social network? Facebook, Plaxo Pulse, and Flickr get this wrong, by my standards, in their automated emails. Twitter gets it right.