Microsoft saw social and search and thought, "we must create our own!" While apple saw social and search, and said, "that's not our market, let's have google handle that for us."
Edit: Apple did acquire ping, but that was social music, which is a bit different. They also introduced siri, which simply plugged in to preexisting search services (Google in iOS 6, Bing in iOS7) so it really is agnostic on that front.
Edit: Apple did acquire ping, but that was social music, which is a bit different. They also introduced siri, which simply plugged in to preexisting search services (Google in iOS 6, Bing in iOS7) so it really is agnostic on that front.