In Britain a john is almost certainly someone called, well, John. There aren't nearly as many of them about as there used to be, but there are still some:
This one is an ex-prime minister:
(That's John Major)
This one is a songwriter and singer.
(Photo by Ernst VikneYes, it's Elton John (who chose his own name: his surname was originally Dwight).
There are even lady Johns:
That's Olivia Newton John
illustration of a demijohn by Pearson Scott Foresman
John Bull is supposed to be a personification of a typical Englishman (though we're really much much wimpier and more laid-back than that image suggests):
Then we have John Barleycorn, who's the personification of whisky, John Doe (a plantiff is an imaginary US legal case which tests a point of law) and John Dory (a fish).
In North America most johns, however, are lavatories. In Australia they're politicians...
...hey, you know something? I think I might be able to see why John as a given name has fallen out of fashion.
Spot the Frippet: john. An early famous John was the cousin of Jesus, and his given name means favoured by God. John Bull comes from the 1712 The History of John Bull by John Arbuthnot. John Dory is named on the same pattern as John Bull. The politician john is short for John Hop, which is rhyming slang for cop (as in being arrested). Demijohn may be something to do with dame-jeanne, which is French for Lady Jane, though what the connection might be I cannot imagine. No one's sure why a lavatory is called a john, but it may be something to do with the very old word jakes, which meant the same thing.