By year 2000, there were "proprietary" messengers aplenty - yahoo, aim, icq, qq, and a bunch more. All of which were simple enough to reverse engineer, so Linux user could just install Gaim (later renamed Pidgin) and connect to most any of those.
What's more, there were already two open standards: SIP/SIMPLE, and newer XMPP.
What's more, there were already two open standards: SIP/SIMPLE, and newer XMPP.