> One of the biggest privacy problems in messaging is the availability of loads of meta-data — essentially data about who uses the service, who they talk to, and when they do that talking. […] the same problem exists with virtually every other social media network and private messenger. Is this true for Signal too? I thought it wasn’t.
Signal does indeed use an architecture (at least for chats with contacts, or optionally everyone when you enable the "sealed sender" option that makes you a bit more prone to receiving spam) where Signal doesn't know who's sending a given message from a given IP address, and only which account it's destined for.
But any entity in position to globally correlate traffic flows into and out of Signal's servers can just make correlations like "whenever Alice, as identified by her phone's IP, sends traffic to Signal, Bob seems to be getting a push notification from Apple or Google, and then his phone connects to Signal, so I think they're talking".