"if Facebook want to remain relevant, they must own a mobile platform. This is going to be Facebook's biggest challenge yet as a company. They are too restricte

"if Facebook want to remain relevant, they must own a mobile platform. This is going to be Facebook's biggest challenge yet as a company. They are too restricted by building on Apple or Google. They must own the platform." uhm..? Why is this? As far as I can tell, Facebook are happy with people using whatever OS and whatever browser to access their site. Why would this be different for mobile?