a) Conversation (https://github.com/siacs/Conversations/ or https://f-droid.org/repository/browse/?fdfilter=Conversation&fdid=eu.siacs.conversations) doesn’t use its own proprietary format of encryption as all the alternatives I know about but uses standard XEPs, so I have no problems to communicate with my wife between Conversation and pidgin using OTR (but Conversations supports more encryption protocols for the cipher connoisseurs). So, whatever alternatives you may suggest XMPP with Conversations (or any other XMPP client) is the widest available most open (in terms of number of independent implementations) option. Which leads to
b) you are putting question in the mutually contradictory way. Either you want the platform where all your friends are, then just switch to using whatever they use and be done with it. If you do not care about security it is Facebook chat most likely, if you do Whatsapp, I guess. If your friends are not using open protocols (i.e., with independent implementations, Signal shows us clearly that FLOSS on its doesn't matter that much), you may get some relief by using system which supports gateways (e.g., I use http://spectrum.im, but any pidgin/Adium around will suffice), but the contradiction cannot be resolved perfectly without sacrificing one or another, because the world in its constant intentional refusal to see existence of XMPP just wants to jump into one proprietary silo after another. Of course, instead of XMPP you may choose another open protocol (IRC and bitlbee), but those are usually even less widespread and with worse user-experience, not mentioning they usually have no encryption whatsoever.
Re: What about conversations?
a) Conversation (https://github.com/siacs/Conversations/ or https://f-droid.org/repository/browse/?fdfilter=Conversation&fdid=eu.siacs.conversations) doesn’t use its own proprietary format of encryption as all the alternatives I know about but uses standard XEPs, so I have no problems to communicate with my wife between Conversation and pidgin using OTR (but Conversations supports more encryption protocols for the cipher connoisseurs). So, whatever alternatives you may suggest XMPP with Conversations (or any other XMPP client) is the widest available most open (in terms of number of independent implementations) option. Which leads to
b) you are putting question in the mutually contradictory way. Either you want the platform where all your friends are, then just switch to using whatever they use and be done with it. If you do not care about security it is Facebook chat most likely, if you do Whatsapp, I guess. If your friends are not using open protocols (i.e., with independent implementations, Signal shows us clearly that FLOSS on its doesn't matter that much), you may get some relief by using system which supports gateways (e.g., I use http://spectrum.im, but any pidgin/Adium around will suffice), but the contradiction cannot be resolved perfectly without sacrificing one or another, because the world in its constant intentional refusal to see existence of XMPP just wants to jump into one proprietary silo after another. Of course, instead of XMPP you may choose another open protocol (IRC and bitlbee), but those are usually even less widespread and with worse user-experience, not mentioning they usually have no encryption whatsoever.