Sometime ppl want to start chat with you. Whether it's about business or a friendly chat, the most important thing is that you don't know them and they left a simple "Hi" comment and went offline. What to you respond?
You can say hi back and wait for them to introduce themselves. No need to worry too much about it. Unless it's a cute girl. In that case you should immediately ask her phone number, address and get a date with her. Lol.