From a previous comment of mine:

To be clear, I want it to be users deciding on Lemmy too. Also, people already here moving to threads wouldn't be the problem, we're small in comparison to them. It would be a few things:

• They would bring in a huge party of users that would take it over and overwhelm the current users. It would be like a cruise ship of tourists taking over a small town and breaking everything for the current residents.
• They could post to Lemmy, but we can't really post to Mastodon. They're going to send ads our way disguised as content, guaranteed.
• If they can manipulate the users from Mastodon, it's going to get out of hand fast. They have teams of devs and psych engineering to accomplish that.
• This is volunteer ran, do we have enough energy to fight Meta when they try to enforce something?
• Can they manipulate Activity Pub software because we're a small team of devs? If they can, they will.
• One person mentioned them having instance owners sign NDAs. What's up with that?