Because there was never a demand for a new social network. Apple is now big enough that anything they build becomes trendy and something people will jump into.
Smartphone market share wouldn't be the defining factor in whether you could jumpstart a social network, given there are billions of active smartphones.
The actual size - not relative size - of your base would define that. The only way relative would matter is if they needed to be larger than eg Android to do it, for some reason (they don't).
They have ~1.4 billion active devices world-wide (900+ million active iPhones). You're acting like they have more like 50 million. They'll soon have a billion people using their various services.
The whole idea of a social network is to connect with your friends. Young people, the most valuable demographic, overwhelmingly use iPhone in both the UK/US. Snapchat/Instagram is a successful social network just controlling this market.
No. Failed because they refused to open up user info. Remember Ping and iAds. Ask the bitter people running those project what they think. To get any user's info you have to go through chains of managers and then a sign off from a board member.
