I was talking to my friend on Facebook. She responed "thnx..." What does this mean?

[melissa]

She said thank you, but wanted to say more that she couldn't express on facebook. An ellipsis is indication that there is more information that isn't there. Although some people don't use grammar correctly, so it's not always used that way especially on Facebook.

She probably just didn't know what to say. In any case, it's difficult to see a persons emotions through texting/instant messaging which is why emoticons were created. Don't assume the worst