SN

SN stands for

Social Media

 * Social Network

IT

 * Surname, an attribute of the LDAP protocol family
 * screenname used in instant messaging

EU jargon

 * Speaking Note

Others

 * Serial Number
 * sine nomine, Latin for "without a name", used in bibliographical catalogs to indicate publisher, etc is unknown
 * "side note" used in messages like "P.S."