Viral­ity isn’t an indis­pens­able fea­ture of all suc­cess­ful appli­ca­tions, but for those where it can be hugely ben­e­fi­cial there are four core prin­ci­ples that help the viral­ity of an appli­ca­tion, says Daniel Tanner:

  • Invi­ta­tion should be a core process, that is essen­tial to using the appli­ca­tion – this will max­imise the chances that your users do invite new users.
  • Keep pulling peo­ple back in, rather than let­ting them for­get you after the ini­tial invi­ta­tion, and make this “reminder” process also be cen­tral to the use of the application.
  • Be use­ful even to the lone user, because that lone user is the source of all your other users.
  • Remove arti­fi­cial invi­ta­tion lim­its, to recog­nise the real­ity that most invi­ta­tions come from a few very active users, and help those users spread the word.

Ten­ner also notes–in passing–the con­cept of the viral loop. Andrew Chen’s take on the loop is the best I’ve read on the topic.