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I would guess that they do something similar to Facebook.

Ars covered what Facebook do about 6 years ago, and even then I think they'd been using it for a few years:

I believe Twitter does something similar too.

Twitter dis use BitTorrent for deploying, in a project called murder: https://blog.twitter.com/engineering/en_us/a/2010/murder-fas...

Not sure they're still using this

MSN Hotmail and Messenger used torrents to distribute binaries in 2006. It's not new nor novel.

And I'm sure someone did it before they did.

What binaries did MSN Hotmail have? It was the earlier web mail incarnation of outlook.com?

The Hotmail front-end ran as an ISAPI filter inside IIS.

Messenger had lots of standalone binaries for different functions: CS, SB, DP, etc.

BitTorrent is used in Dropbox, Twitter, MSN, etc to distribute binaries throughout the server platform, not to end users.