I have installed Signal and do not use it because none of my friends are on it yet. A critical mass is required before I can start using it. You can't migrate group chats from platform X to Signal until everyone in the group is on Signal. This isn't bad news really, as long as people continue installing Signal, eventually I'll be able to start group threads on Signal. I guess it is only a matter of time, assuming more people continue downloading Signal. I think the easier sell is to just get people to install it -- they don't have to use it right away. Then the early adopters can start group threads and chats as they become possible, which will start the shift from other platforms to Signal (assuming people continue installing Signal, of course).