There's nothing to stop Facebook or Google or whoever from spinning up a large Lemmy instance, piling it up with ads and shilling it to their users. As long as it's federated the rest of us could still use non-bloated instances to read/interact with content.

The real risk is companies showing up and injecting bots everywhere. It wouldn't take much for some company to register bot accounts to any open instance out there and spam ads on local and remote communities, making moderating in general more difficult.