People can post on other people's walls. Wall posts are part of the sm:wall collection, as per FEP-400e. Addressing must include as:Public and the wall owner.
Example object:
{
"@context": [
"https://www.w3.org/ns/activitystreams",
{
"sensitive": "as:sensitive"
}
],
"type": "Note",
"id": "http://smithereen.local:8080/posts/110",
"url": "http://smithereen.local:8080/posts/110",
"to": [],
"cc": [
"https://www.w3.org/ns/activitystreams#Public",
"http://smithereen.local:8080/users/4"
],
"target": {
"id": "http://smithereen.local:8080/users/4/wall",
"attributedTo": "http://smithereen.local:8080/users/4",
"type": "Collection"
},
"replies": {
"id": "http://smithereen.local:8080/posts/110/replies",
"type": "Collection",
"first": {
"next": "http://smithereen.local:8080/posts/110/replies?page=1",
"partOf": "http://smithereen.local:8080/posts/110/replies",
"type": "CollectionPage"
}
},
"attributedTo": "http://smithereen.local:8080/users/1",
"published": "2020-05-21T19:05:00Z",
"sensitive": false,
"content": "<p>Test.<\/p>"
}
After someone posts on a wall, the wall owner sends Add{Note} (with a link to the new post) to their followers to signal that a new post was added to the sm:wall collection.
Comments on wall posts work similarly using the sm:wallComments collection.
A note about comments
Since this is modelled after VK, comments aren't supposed to appear in newsfeeds by themselves; they only exist in the context of a top-level post. Thus, comments aren't addressed to anyone's followers. They're addressed to as:Public, the top-level post author, the parent comment author, and mentioned users, if any.