PofM
65
Yes, each server is a separate thing with separate accounts. Sounds like you just gave your mastodon.social login details to the sonomu server admins 
Presumably I need then a separate account for sonomu?
To post on that server, yes. But not to read content posted there
Edit: you can think of it like two separate forums that both use Discourse, e.g. elektronauts and llllllll.co - you need separate accounts to post on each, but you can still read the posts on each site even if you only have an account here. The difference is that discourse forums are just websites and are completely separate things that happen to use the same software. With mastodon you can actually follow users on other servers and repost content from other servers, because the software allows users on separate servers to communicate with each other
If so, how do I wander around mastodon.social AND sonomu using the same client/app?
You can enter the full URL of any (public) post on the mastodon network to view it from another server instance, and then you can read posts (and follow users) from anywhere. It’s just a bit of a manual process to initially “find” content that exists in a separate part of the network.
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