Signal is probably the easiest way for most people to communicate with a very low risk of mass surveillance, and some resistance to targeted surveillance. There are certainly other options for encrypted messaging. Matrix can be self-hosted. SimpleX requires no registration. Both are a more challenging UX for people without a tech background.
From context, however, this is not a problem with Signal’s security, but an informant in the group likely gave the FBI access to the messages.
Is there a meaningful alternative?
It depends.
Signal is probably the easiest way for most people to communicate with a very low risk of mass surveillance, and some resistance to targeted surveillance. There are certainly other options for encrypted messaging. Matrix can be self-hosted. SimpleX requires no registration. Both are a more challenging UX for people without a tech background.
From context, however, this is not a problem with Signal’s security, but an informant in the group likely gave the FBI access to the messages.
Xmpp has been suggested to me.