How push method works when you follow a new user in social media?

I'm building a social media application with a unique feed for each user, like Twitter or Instagram. I have read a lot about the PUSH method where whenever a new activity is created, pointers to that activity is pushed to each follower's user feed.

Now my question is what happens when the user1 just followed another user2. Do I push all existing activities to user1's feed? Or do I only push the new activities after following user2?