Here you see the key performance indicators of the 10 biggest IRC networks per year in comparison. Although the IRC protocol was already developed in 1988, our records begin with year 1998, because this website launched only in August 1998 and we don't have enough data of the time before.
By clicking through the annual statistics you will notice that IRC network QuakeNet had more than 240000 users at the beginning of 2005. This is the highest amount of users that was ever measured within an IRC network. That doesn't seem to be very much compared to the number of users that Facebook and WhatsApp currently report, but you have to keep in mind that we speak about concurrent chat connections. Compared to a well-visited website, that distributes its load across multiple webservers, more than 80000 concurrent visitors, that IRC network freenode currently still has in average (in June 2016), sound not so little. Anyhow... It seems that the Internet Relay Chat has its best times well behind.