Back in the 80s and early 90s, BBSes were ran utilizing phone lines and usually DOS based software applications. To achieve multi-user capabilities, one would have to install multiple phone lines and have a semi-decent computer to provide the users with what they wanted, which was usually door games and files.
These days, though, BBSes can be run on anything from Linux to Windows, and they can have servers for FTP, HTTP, and other internet protocols installed. Mutli-node / multi-user mode is already there since modern BBSes use the telnet protocol. There are still a few BBSes that are in the stone age and are using phone lines for communication. They are few and far between, however.
Fidonet still exists, although it is not the network it was 10 years ago. Fidonet was the largest, and is the oldest BBS network in existance. There were many other networks that have come and gone, but Fidonet is still hanging around.