Fahlman did so on a university online bulletin board, where serious and sarcastic comments were often confused. Online bulletin boards were very well received by the university’s community and were a strong means of social interaction.
In his writings about emoticons on the university’s Web site, Fahlman has stated:
Given the nature of the community, a good many of the posts were humorous (or attempted humor). The problem was that if someone made a sarcastic remark, a few readers would fail to get the joke, and each of them would post a lengthy diatribe in response. That would stir up more people with more responses, and soon the original thread of the discussion was buried. In at least one case, a humorous remark was interpreted by someone as a serious safety warning.
According to EDN