Absolutely astounding to me, petabytes an hour? That's in the region of a meg to several megs per user per hour looking at their monthly active user figures.
Mmm it's not only "telemetry data". It's that (e.g. Scuba) and other types of logs, and not only Facebook (e.g. Instagram as well).
Basically, everything that needs logging and post-processing by both real-time systems (e.g. Puma) and batch processing (e.g. all of the data that's ingested and sent to the data warehouse) goes through Scribe.
Not everything that flows through Scribe is tied to an (external) user, though. Tons of internal systems use it as well, notably anything that logs to Scuba (which is pretty much everything at Facebook. Wide-structured system logs are awesome).
For the curious: https://engineering.fb.com/data-infrastructure/scribe/
Edit: HN thread: https://news.ycombinator.com/item?id=21181982