To gain a follower, then good-naturedly following him/her in return, only so that he/ she unfollows you in a matter of days (2 or 3). A common occurrence among Instagram users, victims are usually busy, proactive people who are not overly conscious of the number of people they follow and follow them.
Frankie: " Oh nice, Mike from class is following me. Guess Ill follow back, cool dude."
* Three days later*
Frank: " Oh lost a follow, wonder who committed this Instatreason..."