Twitter literally interrupts the normal feed with content you haven't chosen. Here, I'll load https://twitter.com to see:

1. dog pic from someone I follow

2. ad ("Promoted")

3. Steve Martin playing a banjo while dressed as a penguin (I follow him)

4. a tweet from an account I don't follow, but a friend liked it

5. a tweet from an account a friend follows, but I don't

6. a tweet from an account a friend follows, but I don't

7. ad ("Promoted")

8. Section of "Who to follow" with three accounts

9. pic from someone I follow

10. a tweet from an account I don't follow, but a friend liked it

11 is an account I follow, and 12 is an ad, but that should be enough to demonstrate. Only three of the first ten items I saw were from accounts I chose to follow, so 70% of the content was supplied by twitter based on whatever they chose.

70%!