In the US, it's Signal. In the UK, it was WhatsApp.

When researchers dumped 100% of Signal's users in the USA, because its contact discovery API has no rate limiting, they found a huge portion of Signal's US userbase has Washington D.C. area codes.

"Signal; Washington D.C. numbers are more than twice as likely to be registered with Signal than for any other area in the US" https://encrypto.de/papers/HWSDS21.pdf

Meanwhile, in Scotland since the pandemic, Nicola Sturgeon ran her government with an entirely parallel communication network on WhatsApp, explicitly to prevent her government's discussions and decisions from being discoverable by FoI requests.

There was daily deletion of messages. It was drummed into people by Sturgeon's head civil servant, Ken "Plausible Deniability" Thompson: https://archive.is/jK6Bd

> Thomson was head of the Covid co-ordination directorate of the Scottish government and wrote: “Just to remind you (seriously), this is discoverable under FOI [freedom of information]. Know where the “clear chat” button is…”. He later added: “Plausible deniability are my middle names. Now clear it again!”

Sturgeon, just like Boris Johnson, retained zero WhatsApp messages: https://www.bbc.co.uk/news/live/uk-scotland-67949454

Scotland only banned use of WhatsApp in government 4 months ago: https://www.bbc.co.uk/news/articles/c4g8pe585z1o

The beltway people working as public servants are (supposed to be) using the TeleMessage fork of Signal. Specifically designed to archive messages for the public record. That is the reason for the increased representation of federal workers.