Here are a few possibilities:
* It could be a typo: Perhaps the person meant to type another emoticon like "XD" or "x.x".
* It could be a personal abbreviation: Some people use unique emoticons in their own personal conversations.
* It could be a symbol in a specific community: It's possible that "xi" has a meaning in a particular online community or subculture.
If you see "xi" in a message, it's best to ask the sender what they meant. They'll be able to clarify its meaning for you.