What metrics would you use to measure notifications at Facebook?

I'd like feedback on my answer to this Facebook metrics question.

FB mission: Build strong communities

Notifications: Help notify users when some friend tags them , posts something, messages them etc so that initates the action for connection. Thus is in alignment with FB mission.

Clarifying questions: There are multiple notifications coming related to Newsfeed, Market place, Dating, Messenger, Groups, pages, Tagging etc. Is there specific area I should focus on?

Answer: News feed related notifications

Objective: Since this feature exists from a log time, engagement and retention will be my area of focus for metrics to evaluate if people find this feature useful and want to use it again and again.

Actions: Users get notified for the following

1. Someone likes/comments to their post

2. A friend FB recognizes close posts something.

3. Someone tags a user in their post- review to show on timeline or not.

4. Someone goes live notification

5. New friend request

On getting notification:

CTR

Replies to friends

Looks through their own post, friend post, friend profile

Accepts, rejects friend request

Watch live posts from friend, particiate in their special moment.

Check timeline, allow,post etc

Metrics:

1. CTR bell icon

2. CTR for each individual notification

3. % of time user reacts to a clicked post

4. % of time user comments to a clicked post

5. % Increased time for session when notifications are clicked.

6. DAU, WAU

7. New friend connections

NS: % of time user takes some action when notified (like, react, comment)

Main metric: 

% Increased time for session when notifications are clicked.

DAU, WAU

CTR