Metric that indicates how many users have saved your tweet as a bookmark
I think this metric is a unique way to gauge long-term engagement among users. Retweets can be likened to quick, instinctive reactions to content, but I think there is value in also providing users with measures that indicate more deliberative reactions (a good way to delineate these user behaviours can be best explained through 'System 1' vs 'System 2' thinking).
For instance, suppose a professor tweets about a recently published academic paper. Whilst retweets provide insight into timely reactions, the academic might want to also keep track of other users who might potentially revisit/avail of their published content in the long-run.