When I started growing more than 20 years ago, ICQ with the OTR (off the record) plugin was the way to go for encrypted instant messaging. Then ICQ changed their protocol which separated the network and proprietary client from the open source and encrypted IM-Clients (like mirandaIM or pidgin). Today the support for ICQ was rebuild for Pidgin and Miranda NG which makes it usable again.
About 10 years ago Jabber with the XMPP protocol was big and is still in use today. A big disadvantage was and is the unreliable server structure, which requires changing the server (when the owner decided to shut it down for example), registering a new username and informing every contact about this change.
Today Wickr and Telegram are more or less widely used, both are encrypted but their client and infrastructure are proprietary. Wickr was acquired by amazon 2 years ago, so it is in the hands of the US-Government and Telegram already sold out their users to the German Government (political opposition). Sure there is Signal (another Instant Messenger) but as they require the phone number for registration, the anonymity is wasted right from the start, same applies to telegram, icq (or you have an old icq-account, then you don’t need to create a new account).
Here I have compiled a small list where with pros and cons.
ICQ: Encrypted messaging through proprietary infrastructure, requires a phone number for new accounts
Jabber/XMPP: Encrypted messaging through open infrastructure, when your xmpp server goes out of business you need a new one and inform everyone abut that change
Telegram/Signal: Encrypted messaging through proprietary infrastructure, requires a phone number for registration, so no anonymity right from the start which makes the encryption senseless
Wickr: Encrypted messaging through proprietary infrastructure
WhatsApp: You joking right?
What IMs do you use, or what IMs should anyone use when he/she wants to interact with OG members?