Answer:
The number of users or monthly market share of users
Explanation:
A brand manager would be primarily interesting in the potential market of each social media, in order to decide in which of them to put ads.
For this reason, the brand manager would primarily focus on the number of users of each social media, or on the monthly market share of users.
The manager could also try to be more specific, and assess the number of users of the target audience for its brand. For example, if the brand is women's clothing, the brand manager would obviously only focus on the number of female users.
