All of them release numbers on this:

https://www.apple.com/legal/transparency/

https://transparencyreport.google.com/user-data/overview?hl=...

https://www.microsoft.com/en-us/corporate-responsibility/law...

https://transparency.fb.com/data/government-data-requests

The subpoenas are for cloud stored data.

Downvotes are likely by people who read the article:

> “Most shocking is just how routine secrecy orders have become when law enforcement targets an American’s email, text messages or other sensitive data stored in the cloud,” said Burt, describing the widespread clandestine surveillance as a major shift from historical norms.

So it's mostly communications data stored in the cloud. We know Apple and Google receive similar requests because they also release data:

https://www.apple.com/legal/transparency/us.html (US: 4600 devices in Jan-Jul 2021)

https://transparencyreport.google.com/user-data/overview?hl=... (US: about 60,000 in 2019)

Apple turns over customer data on over 30,000 users per year without a warrant. It's in their transparency report under FISA orders.

No probable cause is required.