If you think of it more like Twitter, you can post like a blog, follow people's posts, or send direct messages to users. Then it is like a few dozen Twitter websites ran by different people, each with their own rules and own little user base. Then they connect all those Twitters together, so people can follow, be followed, and message users on other, 'foreign', Twitters, making the dozen separate smaller Twitter websites into a large federation of many. To help you meet and friend/follow people from foreign Twitter websites, they usually have a mingling area, with like a global feed from all of them. They usually call this "federverse" or some name based on the word federated.