They all serve different purposes.
For example Mastodon is for "microblogging" or basically what you do on twitter. Everyone yelling out a few SMS messages worth of stuff and others can reply to it.
Lemmy is a "news aggregator" or basically what Reddit does. People post on a topic and others create threads of discussion on the topic.
These two services arent serving the same purposes. What they share is an underlying protocol for doing federated social networking.