How things are different

• most instances are run by non-profits / without ads
• the fediverse is designed to prevent individual sites from growing too large and monopolizing a form of social media. If an instance starts to become what reddit is today, users can migrate to another instance: https://fedecan.ca/en/guide/get-started

I've seen users called "Lemmings", but usually it's just "users". There are other interconnected platforms such as Piefed and Mbin, so "lemming" doesn't cover it well. "Fedizen" is another more general term

For growth stats see here: https://fedidb.com/software?type=forum

For differences:

• there's less content here, and no algorithm trying to keep you engaged. It's up to you how much you want to use it. If you want more content, I recommend subscribing to a lot more communities, and then sorting by scaled or something: https://fedecan.ca/en/guide/threadiverse/finding-communities
• because of how the platform works, upvotes/down votes/mod actions are public record. This reduces vote manipulation and shenanigans, or at least lets people deal with it after the fact: https://fedecan.ca/en/guide/threadiverse/privacy-lemmy
• grab a mobile app, Reddit used to have lots of options too: https://fedecan.ca/en/guide/threadiverse/apps

Welcome :)