Actually, I'd say the Clacks was a semaphore system. Before telegraphs were invented, semaphore was used fairly effectively. It was certainly used in the Napoleonic Wars.
Where the Clacks is like the Internet is in the fact that it is a revolution in communication for the Discworld.
For the first time people are able to contact each other from one side of the Disc to the other is a reasonably quick time.
It doesn't work like the Internet, but it does bring people closer together in much the way that the Internet does.
I think Terry is using it as a parody of the Internet in much the same way that iconograph devices are parodies of cameras and the Gooseberry in Thud is a parody of Blackberries in our own world.
“Men never commit evil so fully and joyfully as when they do it for religious convictions.” – Blaise Pascal