"laterally related market segment." Facebook's core competency is social networking. Mobile devices are one mode of social networking - it is a logical choice

"laterally related market segment." Facebook's core competency is social networking. Mobile devices are one mode of social networking - it is a logical choice of theirs to create their own mobile device, strengthened by their brand it would be adopted on novelty alone; to a global user-base they can reach at little cost seeing as how they own their own advertising platform.