I want to know is there any criteria through which sites get listed in footer? I am asking this because bicycles seems to be a trivial topic to be listed.
All of the sites that were either part of the original trilogy (Stack Overflow, Server Fault, and Super User) and those that have graduated the Area 51/Beta process are listed in each other's footer. Careers in also included in this bunch. Bicycles went through the phases of the Area 51 system and graduated 11 months ago.
All of the sites currently in Beta are listed in the footer of all of the other Beta sites.
Area 51 itself is listed in both the footers for the Beta sites and the established ones.