I don’t think pitching #ActivityPub to existing social media makes sense. Adding federation to a non-federated social media service isn’t a net win.
You have to spend the time and money to implement it. Then you have to spend the time and money to maintain it. Most of the time, ActivityPub support is implemented as mastodon compatibility, not true AP support. This means having to constantly make sure you keep up with masto changes and constantly fielding issues with other implementations because you didn’t fully implement AP.
And after implementation, you don’t just gain access to a ton of new users, you have to take on the burden of moderating all of it (which is a persistent ant recurring time and money cost). And since the #fediverse has a ton of opinions on moderation, you’re always pissing somebody off.
And after all that, what you’ve enabled is an easy way for your users to recreate their social graph without your service. The idea of an interconnected social web is cool, and hopefully it’ll be the futrue, but it doesn’t make sense for profit-driven businesses.