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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
You'll want to use /r/{subreddit}/about (official API docs here, and over here for some unofficial docs on what is returned). The field on the object that is returned that you're looking for is user_is_subscriber.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.