If ActivityPub gets really popular, there won't be a single service that's winning. For example, if each Youtube profile is suddenly a fediverse account which you can follow, the owner of that Youtube profile doesn't need a separate Twitter or Wordpress account - simply the act of uploading the video to Youtube will appear in user timelines of twitter, mastodon and all other services. Same would happen with blogs, instagram accounts, etc.
And if we get to such future, it's possible that we won't even need a Mastodon/Twitter account to have a timeline. iOS and Android could build native support for following accounts and displaying timelines in the operating system.
And if we get to such future, it's possible that we won't even need a Mastodon/Twitter account to have a timeline. iOS and Android could build native support for following accounts and displaying timelines in the operating system.