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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.
I know of Smithereen[1] that is an attempt at a full fledged social network similar to VK on top of the ActivityPub vocabulary.
[1] https://github.com/grishka/Smithereen
NOTE:
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Hence, a higher number means a more popular project.