Apple has an insider advantage. They just need to be careful about how they use it. Apple needs to keep adding social aspects to its existing apps rather than build an entirely new social media app.
They've started doing that with stickers and doodles in iMessage which is already a social app. They could take that further by allowing iMessage users to create profiles for themselves and to post iMessages that are visible to all of their followers, instead of to a specific person or group. That becomes their "wall". iMessage is the best existing app to form the foundation of an Apple social network. It's already very widely used and I suspect that it already competes favourably with Snapchat and FB Messenger for attention.
The Photos app already allows you to share albums with others and for those people to like and comment on your photos. This feature is too hidden. Instead, in iOS 11, the Share tab needs to be rebuilt and maybe rebranded. Allow users to post photos to this tab which would be accessible online as a website where others could follow you (your choice of public or contacts only).
Other apps that can become more social:
Music: allow users to subscribe to their friends' playlists and to see their likes.
Activity: already becoming social with the latest addition of sharing. Add challenges next.
Safari: Add a button to share a website with room for commentary
Contacts: this is where you would manage your followers and who you follow.
Calendar: add a broadcast feature to post events to all of your followers.
Once all of these apps are social, a new app could become an aggregator where you see all of your friends' activity — essentially an app that acts like Facebook's newsfeed. Everything they post in photos, their public iMessages, their Apple Music likes, their shared Safari sites, etc.
