[sliken]

If two users PGP email to each other, anyone who's monitoring either mail server or the network in between can tell A) when they are talking, B) who they are talking to, C) what the subject of the email is, and D) how big the email is.

With signal this is much harder, for instance google/apple do not know who you are chatting to. They also don't handle the encrypted message transport/delivery.