On forums, IRC, blogs, and the like I realize that the time of the other person is valuable. Therefore starting with any kind of greeting (or signing posts/messages/whatnot) is an insult and a waste of their time.
Blogs, forums, facebook, email: all of that includes identifer information so signing messages is a redundant waste.
Same for the greeting. On most IRC channels any kind of greeting is often met with silence, even if the channel was fairly active before the greeting is thrown. The way to get an IRC channel to respond is to ask an intelligent question that piques the interest of those in the channel that may know an answer. Forums, blogs, email, facebook: these are all similar, with the single possible exception of formal business emails. With the internet being such a source of information overload, the last thing I want to have to parse past in emails, blogs, and forums is useless greeting cruft.