Cool discussion.
Funny, to me there is no real difference in nature between Solid and ActivityPub. I’ll have to read the ActivityPub spec again to be more assertive in this conversation, but I believe ActivityPub is actually a subset on Solid, both based on Linked Data Platform.
It feels to me Solid tries to standardize any kind of web interaction, whereas ActivityPub basically standardizes Twitter (I hope I’m not insulting anyone here, I love ActivityPub).
Both initiatives aims to break free from data silos and feel very much compatible with one another to me. I’ll read the specs again this weekend to make sure I’m not saying something totally wrong.