That’s not entirely true- you can upload your own, but you can only seed to users that do have port forwarding. On many trackers, that initial seed is all going to seed boxes with an autograb script enabled anyway, and those do have port forwarding.
If any instance becomes large enough to have an undue influence, which Meta would likely have, then they effectively control the entire ecosystem. At that point, it effectively stops being decentralized (See: The 51% Attack, although this wouldn’t happen at a certain number/ratio). When it becomes convenient to them, they can pull the plug, and destroy the rest of the ecosystem that isn’t theirs.
It’s exactly what happened with XMPP and Google Talk.
The numbers are highly skewed because of the launch. A number of users are being paid to create content during the launch. A lot of the users are just checking out the hype. Some will stay, many won’t.
The numbers won’t really be useful or comparable until the dust settles. I give it a month.