Fandom loves an abbreviations and "Supernatural" obviously has far to many letters in it.
In the early days of the fandom SN was used on things such as Livejournal community names such as the sn_newsletter. However some fans found it could be confused with the TV show and fandom for "Sports Night", while SNL is associated with Saturday Night Live.
SPN became the more widely used abbreviation. Its usage spread even more widely with the advent of the letter economical Twitter
SPN (pronounced "ess pee en") is usually capitalised and is used to refer to the show and the fandom.
- See also Super Fans