This is probably an ignorant question on many different dimensions, but as a newbie to this world of users controlling their own data – and encouraged by it – I’m trying to get the lay of the land.
What’s the relationship of Solid to the Fediverse and its apps like Mastodon and Diaspora?
Thanks.
Not a dumb question, don’t worry. Some confusion arises as “Pod” is a term that is used to refer to a Fediverse instance and the personal Solid data store as well. But these things are acually completely unconnected right now 
Although there are people interested in bringing these two together. (I’m one of those)
Technical background: Solid provides the underpinning so that people can store their data in their own place and give access rights to it to other people and apps. Solid is not really a presentation layer - so you cannot browse things, and there is no timeline that shows you the content from all your friends and people you want to follow.
The Fediverse is a set of web servers that store your data and handle the timeline and all that.
It is entirely possible to create a Fediverse server that instead of handling the presentation and the data storage just does the presentation layer and then stores all the data on your Solid pod.
You can add this thread to the topic http://forum.solidproject.org/c/connection-to-other-initiatives
Some more newby statements that may be relevant in this regard:
I believe moving your account (including followers, toots, etc.) from one federated (ActivityPub) server to another is still not possible and well-defined. May be a use case for Solid there.
The ActivityPub standard leaves authentication / authz to implementers, advises OAuth2, I believe (no time to check now). Another use case?
Also the solid-spec mentions Followers and Following as TBD, and these are also part of ActivityPub
Solid-spec also has a link Feed Discovery in the ToC for which there is no paragraph text, but these are also AP / ActivityStream concepts.
Upcoming PixelFed may have overlap with, complement, or be a competing alternative to what @timbl is suggesting in Photos manage my pictures/images/slides? (and ImageSnippets mentioned there).
Did you look to the Zot Protocol ?
Specs
Intro
It seems to me that the concept of nomadic identity fits really well with Solid pods.
If that is the case it would be very interesting to the people building ActivityPub apps for the Fediverse. An opportunity…
FYI, referencing Github issue related to ActivityPub in node solid server (the label is ‘revisit’ so the issue is not really closed):