This is quite an interesting question, really. Let's see how I'd go about it.
Alright, so the problem boils down to this: how to notify a user of any changes live from the people he's following. Okay.
So, let's break this problem down: a user can have a unique ID; and whenever s/he posts something, a small JSON object is pushed to a queue:
{
"id": <GUID>
"user_id": <USER_ID>
"item_id": <ITEM_ID>
}
Here, item_id can be the item (image, post, etc.) itself. Now, whenever a user posts something, this gets queued. And when the queue gets this object, it sends out a push notification/publish-subscribe message to the followers the person has. On receiving the message, the message is displayed in the user's feed.
Now, I would like to say that publishing the item_id would be better, and then later, fetching the entire data would make this system faster.
There are other methods to do this as well. I would love to design one if you want me to! :)
I hope this helps!