netsplit.de provides extensive information about the Internet Relay Chat (short: IRC), a chat protocol that was already developed in 1988 and that's internationally used for text based communication over the Internet. On IRC you can directly chat with individual persons via private messages or you can enter chat rooms (usually called channels) about many different topics. Within those IRC chat rooms you can easily find chat communities that consist of several hundred people. (Please have a look to freenode channels or QuakeNet channels, if you don't believe yet!).
View statistics and small reports about registered IRC networks and IRC channels, get informed about their users and topics and enter a chat room of your interest with your browser via irc2go! The following graph gives you a little impression about the amount of concurrent users and channels that were seen on the Internet Relay Chat during the last week.
PS: Do you want to know more about IRC's history and structure? Then you should take a few minutes to ask Wikipedia about the Internet Relay Chat.