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 씨
or 님
to the other user's name?
3 Answers
님 is practically the standard way to address people's usernames on the Internet. I've never seen/used 씨 before, though.
님 is usually associated with some kind of profession, occupation, or title while 씨 is usually used after a person's name. Hence it's not wrong if you address someone by his/her username follow by 씨.
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.)