Assumption: users use FB mainly for socializing with their friends.
This is an on-going existing problem, which was not caused by specific events or incident.
Hypothesis 1: The new user has not formed a specific habit of using the app or she forgot to use the app.
Hypothesis 2： After the user registered, she does not feel the need of going back to the app
Hypothesis 3: the user is simply not a target user. The user kept going back to the competitor app such as snapchat for social networking.
Data to be collected:
(1) demographics data about these group of users to understand if they fit into FB's majority user profile. For example, age is an important factor. Teenagers tend to use snapchat. Younger age implies that the users are simply not the target users for FB. Therefore, the solution should be more at the strategic level - whether or not FB should study the competitor's features and customers and enter new territory by developing new features/products/buying other apps
(2) Study those users' friends' usage of FB and separate them into different buckets. If the user has a group of highly actively friends on FB, the user might just forget to use the app or need a little bit more influence from their friends. A reminder email to the user or even remind the user's friend on FB to say hi or interact with the user on FB might help solve the problem.
If the user's friends are not active on FB, then, in general, the user might find FB is not necessary for her to socialize with her friend. Therefore, FB might want to target other than socializing features such as pushing random news or random interesting content to get the user's attention on FB's other functions.
Post launch of the solutions, tracking user's login behavior, CTR of reminder emails, content they tended to consume, features they use most, interactions with their FB friends, growth of FB friends etc.