My examples are all based on the CIA and NSA playbook though, as it was the NSA director that said the quiet part out loud, explicitly, in front of Congress. The NSA is effectively America's red team, an offensive arm, meaning they (should be) focused on threats (percieved or otherwise) outside the country
The FBI has been much quieter about this though - there has yet to be a Snowden-for-the-FBI, though they would be one of the agencies I would fully expect to be doing similar work domestically.
As this becomes more well-known, I would expect state and county police to start looking into data and metadata as well. In some cases, they already are [0] - even if some aspects of that case are less relevant today (Google Maps no longer uploads location history, though cell tower trilateration is getting more accurate, not less).
It's far more prevalent than most people realize, though I invite you to consider which you'd rather have when building a second-by-second profile of a person's life: the message contents, or the metadata?