씨: a common title like Mr, Mrs, and Miss. Always used after a name, which may be a full name, last name, or first name (often for one's love interest). 님: has many uses, and in all cases more polite and respectful than 씨. 1) as an honorific for titles - comes after the title word 임금님, 대통령님, 의장님, 사장님, 부장님, 과장님, 장군님, 하나님, 아버님 .. 2) pronoun (used by itself) for someone one loves, worships, or respects highly - mostly used in literature. => 님은 가셨습니다 / 님께서 돌아오실 날 까지 / 님을 향한 마음 3) just like 씨: connotes greater respect. Usually comes after a full name. => 김영희님, 홍길동님 4) as "you" (by itself), like on the Internet - this is mostly because there's no other good word for "you". => 님께서 말씀하시길 ... (You previously said that...) So they might be using 님 online in the sense of #3 or #4. #2 sense is usually reserved for writing only, but you know on the Internet they write lots of things out of the norm, so who knows, they might have been writing it like in the literature. "시" you saw was probably a typo. I've never heard 시 used like 씨 or 님.

shi (씨) can be translated as Mr./Miss/Mrs. When you use this title, you are showing a level of respect. Shi (씨) is attached at the end of the name. We’ll use Minji (민지) as the first name. Minji shi (민지 씨), means Miss/Mrs. Minji. (Minji is a typical name for a girl). Using shi (씨) also indicates a level of formality. This would be used towards strangers, but not friends. Nim (님) is also a title of respect. Nim (님) is usually attached to professions. The difference between nim (님) and shi (씨) is that shi (씨) is used after actual names, while nim (님) is used after an occupation. The word for teacher (선생님) is a word that often has the nim (님) attached. It’s not only the professions that attach nim (님). The word for god (하나님) also contains this honorific title, so do family members like grandmother (할머님). Source : http://blogs.transparent.com/korean/honorific-titles/