- number of times questions from a certain time span (e.g. from a given day) were tagged as a favourite,
- number of comments for questions from a certain life span,
- number of views of questions from a certain life span,
- number of replies for questions from a certain life span.
Perhaps it's me, but questions (on SO) could be indicative of poor documentation, or a lack of answers elsewhere. The latter, obviously, somewhat counter to popularity.