It depends on the people, the conversation, the tone, etc.... On this forum you will notice that sometimes people use it when referring to other posters/contributors, and other times they don't. Using it adds a politeness, a civility, a formality to the conversation - which is nice to have sometimes. Not using implies a familiarity, casualness, or it can just be omitted due to this being a forum of largely English-speaking people who do not have such a linguistic convention, and so it is left off with no implied meaning whatsoever. There is no set rule for online chatting, but my default position would be to add it until/unless you notice it is not being used by other members.