Why have the majority of social networks decided that people using their service in the most basic way deserves a notification?

"Ooh, I have 3 notifications according to the app icon, they must be things that need my attention."
<opens app>
* In case you missed DickSlap's [tweet/post/other]
* RomanHelmet liked a [tweet/post/other] from SpaffMuncher
* MuffSnapper added a new photo

Why does that need a notification? That's just users using your app. That's what I expect when I open the app. I expect to see people posting things. This is going to make me start ignoring notifications in the same way I ignore emails marked as urgent.