If one wishes to be polite in reference to another user met in an online forum (e.g., StackOverflow or a Facebook Group), is it appropriate to add
님 to the other user's name?
Sometimes 씨 can be used between people of "equal" level (say, between colleagues at work). However, using 씨 to address someone you don't know well can easily sound impolite/belligerent, especially online.
On Internet, people mostly just use 님, even though, strictly speaking, (I think) it is against the language standard. (Traditionally, 님 could only attach after personal title or profession (임금님, 할머님, 선생님, 교수님, 손님, etc.), but not after a name. Well, shrug.)