You aren’t asking for perfect security so I'll just mention Telegram. I use it with my friends and family after WhatsApp betrayed us. Works really well for us

You aren’t asking for perfect security so I'll just mention Telegram. I use it with my friends and family after WhatsApp betrayed us. Works really well for us but isn't like Facebook at all. The differences is it isn't broadcast-focused or build-huge-networks focused but is more organic.