Some people love saying Mastodon/OStatus is very scalable because of federation but let's be honest that's some bullshit. It's easy to split and multiply, but that's not scaling.
The birdsite is much easier to scale.

Just imagine one edge case we're not seeing for now: many people using it (let's say millions) and following a very famous person
That person must have their own instance at this point, at least.

... what happens when they toot?
just imagine the enormous peaks and lag it'll do. And we're not even talking about media.

How would Mastodon realistically handle Katy Perry? I have no idea.

just in case someone read it later: the real question is not really technical but: How do you convince Katy Perry to join Mastodon and pay for a high-end instance and a full-time sysadmin and more CM when burdsite is free?

@CobaltVelvet only a small percent of the community of an instance needs to contribute for the rest of the community to benefit

17 people on cybrespace are donating so that 223 others can enjoy it without needing to pay a cent