There are legitimate circumstances in which you'd use 3rd person pronouns in a forum like this. Whenever you refer to another user (as in someone else than you're directly replying to), you'll probably use that person's username the first time, but then use pronouns for all repeated mentions as long as context allows, especially in the same sentence. So knowing people's pronouns makes it much easier to write well flowing sentences.

Unless that user explicitly asks not to use pronouns, in which case don't.