No Hello

Why?

There are multiple reasons:

• Time: if you say "Hello", I say it back, then you write your question I will have to wait for your question to be typed after I've already interrupted what I was doing to read your simple hello. If in addition to that, I also go back to my task before you're done writing your question, the time used to to finally get your answer becomes at LEAST twice my response time.
• Less context switching: Every time someone stops their task and reads a chat, they will need time to go back to their task and sometimes they might forget what they were doing in particular.
• Less pressure on the receiver: This might not apply to everone but SOME people might feel pressured to wait for your second message after the hello because the answer to your first hello might feel like a commitment of "being available".

What do I send then?

Depending on what your intent is:

• Just chatting: "Hello, I found through <PLACE> and I'd like to get to know you!" or "Hey, I'm ... I found you in <PLACE>, what do you think of ... ?"
• Asking a specific question: "Hi, do you know ... ?"
• Needing something in specific: "Hello, I'm here to ..."