Facebook Messenger was (is still?) based on the MQTT protocol https://mqtt.org/2011/08/mqtt-used-by-facebook-messenger https://mqtt.org/2011/08/mqtt-used-by-fa

Facebook Messenger was (is still?) based on the MQTT protocol https://mqtt.org/2011/08/mqtt-used-by-facebook-messenger https://mqtt.org/2011/08/mqtt-used-by-facebook-messenger that is specifically designed to be forgiving of low bandwidth or intermittent connectivity. It's a great example of a company picking the right tool for the job instead of falling victim to not-invented-here syndrome.