What metrics would you track if you were PM of Facebook Birthdays?

Clarifying the feature: As far as I know, the Facebook birthday feature was introduced as way for people to wish their friends on their birthdays in a memorable and fun way, with a little personal touch without much heavy lifting. Facebook as a company strives to connect people and birthdays are traditionally important milestones where acquaintances re-connect particularly if they do not meet each other often.

Feature Goal: The feature enables people to connect with each other and therefore contributes to Facebook's overall goal of connecting the world. As the individual connections become stronger, the social network becomes more valuable to its users as a place to gather, for Facebook as its user base becomes more engaged and to Facebook's advertisers customers due to engaged and large user base.

My understanding of - How does it work?

Typically, people forget about birthdays unless it is about people that are really close to them in day to day life. With so many friends, it is difficult to track, difficult to say anything meaningful and ultimately grow the strength of the connection on a meaningful moment in someone's life. Facebook asks its users to provide their birthdates as they become members. When someone's birthday comes up, Facebook members get a notification about an upcoming birthday, as well the invitation to create a digital card with personal memories as part of that. This replaces the typical two words "Happy birthday" people would write on their friends timeline. However, with this personal touch, it is much more interesting for the recipient to go through the birthday wishes and re-acquaint / renew their relationship. This leads to more sharing and communication and grows the relationship.

Metrics:

The main objective of the metrics would be to understand if the Facebook birthday feature is achieving the results desired above: stronger relationships across a significant number of Facebook audience.

Brainstorming the stages:

Awareness

1. First the users must know about birthdays - this can be measured by the total number of users who had oppotunity to wish someone in a given time window - say a month for now.

2. Second the user must wish their friend on the birthday - % of the total users who has the opportunity, who actually interacted with the birthday notification say with a simple note.

Engagement

3. Third, the user must decide to create a memorable experience - % of the total users who actually created a memorable experience instead of a simple note.

4. Fourth, the recipient must engage positively with the experience - % of users who received a personal experience and engaged with the birthday wish. This can be compared to % of users who engaged on receiving a normal note.

Engagement with Facebook - the overall product

5. The level of connection improves as a result between the friends   - a casual experiment could be run to demonstrate that the indicators of relationship - such as messages / engagements between friend who have been exposed to the birthday experience v/s not.

Referral

6. Further, there is an inherent virality about such messages. If someone likes the experience, they are likely to share it. Percent of users who received a birthday wish via FB memorable experience for the first time and had an opportunity to wish someone else within a time window, used the Facebook Birthday feature with one of their friends.

Retention

7. Repeat users - % of users who are repeat users of the feature within a time window. This indicates that the user was succesful in conveying a personal message to their friend last time,

Comments and critiques much appreciated. I feel like I am missing something in the structure here.