how to greet someone who's a complete stranger and just met online?

Well when talking with online people until you realize if you are really meant to be friends, a lot of times it is best just to talk about interests, or where you are from, etc. getting to learn about each other a little first and see if you can develop a Friendship or not as well. You might not want to share too much personal information yet, but sharing things about you can help you learn more about them usually as well.

When you are online friends you must have at one thing in common and that is the website where you met eachother. Did you chat together? Did you exchange emails? And what did you talk or write about online? I would ask him some questions about the things he has already told you. If he told you he had just gone on a trip, gone to a concert, started a new job etc ask him about it and tell him about your own experiences. That is how I usually do when I meet online friends. Good luck with the meeting