What is the purpose of Facebook Timeline?
Facebook Timeline helps users in the following ways -
a. View a chronological list of timeline posts which has been posted by them or addressed to them.
b. Create Posts, Milestone etc.
b. Update Profile Information
c. Launch and Organize Photos, Friends
If Facebook newsfeed is the place where users see what is happening in their friends life, Facebook timeline is the place where users can organize and manage their personal life and curate what they want to tell the world.
Coming to the question, the next step is to identify if the question is meant for one particular segment of customers or for all the customers. Let us assume it is for the entire customer base of Facebook. Retention metrics consists of measuring behaviours of users in timeline and the success of that can be measured as follows -
a. Number of timeline page visits/user/day
b. Median time spent in the timeline/user/day (Measure active time spent in the application. Active Time is the time spent where the user has scroll, clicked or performed any other activity in the page for greater than 1 minute) - Look at 20 percentile, 50 percentile, 90 percentile and 95 percentile metrics for this would give a sense of how long users spend in the timeline page and track retention for the Low, Medium and High Users of Timeline
c. % Time Spent in Timeline vs Rest of the application
d. Posts Created via Timeline/ Posts Created via Newsfeed (Measure this metric by each kind of post - video, life event etc.)
e. Comments, Likes , Shares from Timeline per day
I would prioritize b, e, d, c, and a metrics since those metrics are the key indicators which denote positive behaviours of Timeline users.